(19)
(11) EP 3 804 553 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication:
14.04.2021 Bulletin 2021/15

(21) Application number: 19202573.2

(22) Date of filing: 10.10.2019
(51) International Patent Classification (IPC): 
A44C 17/00(2006.01)
(84) Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
Designated Extension States:
BA ME
Designated Validation States:
KH MA MD TN

(71) Applicant: D. Swarovski KG
6112 Wattens (AT)

(72) Inventor:
  • Blasbichler, Günther
    6020 Innsbruck (AT)

(74) Representative: Moore, Michael Richard et al
Keltie LLP No.1 London Bridge
London SE1 9BA
London SE1 9BA (GB)

   


(54) CUT FOR A GEMSTONE


(57) A gemstone comprising a crown and a girdle at which the gemstone has its largest transverse dimension, the crown comprising first set of main crown facets and a second set of crown facets meeting at an apex of the crown; and optionally a third set of crown facets inclined relative to the plane of the girdle and alternating with the first crown facets around the perimeter of the crown. Methods of improving the optical properties of a gemstone and articles incorporating the gemstone are also described.




Description

Field of the Invention



[0001] The invention relates to a cut for a gemstone, and in particular to a cut for a flat back gemstone. Articles comprising the gemstone and methods for improving the optical properties of a gemstone are also provided.

Background



[0002] Faceted gemstones have been used to embellish products for a long time. The optical properties of gemstones are particularly important characteristics in this context, and these properties are defined at least in part by the geometry of the gemstone. The optical properties of a gemstone are often compared to those of a gemstone with a brilliant cut. The brilliant cut is a complex geometry with four different types of crown facets (including a table) and two different types of pavilion facets, all of which interact to create advantageous optical properties such as brilliance.

[0003] In some circumstances, it is advantageous for gemstones to have a flat back surface. Such components can be combined with a mirrored surface on the flat back in order to create a "sparkling effect" where light incident on the mirror surface is reflected in multiple directions due to the interaction with the facets of the component. However, such flat backed gemstones typically have inferior optical properties compared to e.g. a brilliant cut, due to the absence of pavilion facets, even in the presence of a reflective layer.

[0004] It is against this background that the invention has been devised.

Summary of the Invention



[0005] In a first aspect, the invention resides in a gemstone comprising a crown and a girdle at which the gemstone has its largest transverse dimension, the crown comprising a first set of main crown facets inclined relative to the plane of the girdle and adjoining each other around the perimeter of the crown, wherein the crown further comprises a second set of crown facets meeting at an apex of the crown. Thus, the second set of crown facets can be considered to define the apex of the gemstone.

[0006] In a second aspect, the invention resides in a gemstone comprising a crown and a girdle at which the gemstone has its largest transverse dimension, the crown comprising first and second sets of main crown facets inclined relative to the plane of the girdle and alternating around the perimeter of the crown, wherein the crown further comprises a third set of crown facets meeting at an apex of the crown, while two sets of main crown facets (the 'first' and 'third' crown facets) alternate around the perimeter of the crown. Preferably, by the term 'alternate' it is meant that one first main crown facet is directly adjacent one third (main) crown facet and this relationship repeats around the perimeter of the crown; i.e. each third (main) crown facet is located between adjacent pairs of first main crown facets and vice versa.

[0007] The present inventors have surprisingly discovered that the inclusion of a set of crown facets meeting at / defining an apex of the crown (termed herein the 'second set' of crown facets), instead of a table (which is common in the art), results in a gemstone with particularly advantageous brilliance properties. These advantages are particularly salient when the gemstone has a substantially flat back.

[0008] According to the second aspect, the second set of crown facets is spaced from the girdle by the first and third sets of main crown facets. Similarly, according to the first aspect, the second set of crown facets is spaced from the girdle by the first set of main crown facets.

[0009] In embodiments of any aspect, the apex is an apex in the strict geometric sense of the word, or the apex may comprise a small flat or slightly rounded region ('culet'). In some such embodiments, the small flat or slightly rounded region of the apex represents less than 5%, less than 4%, less than 3%, less than 2% or less than 1% of the surface area of the gemstone when viewed from above.

[0010] Typically, in embodiments of any aspect of the invention, all facets in a set are substantially identical. Within the context of this disclosure, facets may be considered to be identical if they have substantially the same shape and are arranged at substantially the same angle relative to the plane of the girdle. In asymmetric embodiments of the invention, however, facets of one type may vary in shape as discussed herein below.

[0011] Within the context of this disclosure, the plane of the girdle is an imaginary plane relative to which all of the facets within a set of facets typically have the same orientation and distance.

[0012] Suitably, gemstone embodiments of any aspect of the invention comprise a substantially flat back. In some such embodiments, a reflective layer is provided on the substantially flat back to reflect light incident on the reflective layer through the facets of the crown.

[0013] In embodiments of any aspect, the flat back is substantially parallel to the plane of the girdle.

[0014] The present configuration of the crown of a gemstone according to the invention is particularly beneficial in the context of gemstones with a flat back, where the light return and fire may be relatively low due to a lack of pavilion facets.

[0015] However, in some embodiments of any aspect, it can be envisaged that the gemstone may comprise a shallow pavilion. For example, the gemstone may comprise a pavilion that has shorter height than the crown: for example, at least twice as short as the crown, and preferably at most about 20%, or at most about 10% of the height of the crown.

[0016] In embodiments of any aspect, the girdle has a height of no more than about 33% of the height of the crown, no more than about 25% of the height of the crown, no more than about 20% of the height of the crown, or no more than about 15% of the height of the crown. In embodiments, the girdle (when present) has a height of no more than about 15% of the diameter of the gemstone and preferably no more than 10% of the diameter of the gemstone. Suitably, the girdle is substantially circular. Circular gemstones are commonly used for their optically pleasing symmetry.

[0017] In embodiments of the second aspect, the facets in the second set of crown facets adjoin the facets of the third set of (main) crown facets. In embodiments of the first aspect, the facets in the second set of crown facets adjoin the facets of the first set of main crown facets.

[0018] In embodiments of any aspect, the facets in the first set of main crown facets adjoin the girdle with a broad side.

[0019] In embodiments of the second aspect, the facets in the third set of main crown facets (which alternate around the circumference of the girdle with the facets of the first set of main crown facets) adjoin the girdle with a point.

[0020] In embodiments of any aspect, the facets in the second set of crown facets are substantially kite-shaped. In some such embodiments, the facets of the second set of crown facets adjoin each other with their long sides.

[0021] As the skilled person would understand, kite-shaped is intended to define a quadrilateral whose four sides can be grouped into two pairs of substantially equal-length sides (a pair of relatively short 'minor' sides and a pair of relatively long 'major' sides) that are adjacent to each other.

[0022] In some embodiments of any aspect, adjacent sides of a facet may, intentionally or inadvertently, not meet at a perfect vertex. In such embodiments, the respective facet(s) may comprise one or more additional short side separating the pair of adjacent longer facet sides. In such cases the short side is typically significantly shorter than the adjoining longer sides, and may for example have a length of between 0 and about 10%, between 0 and about 5%, or between 0 and about 2% of the length of the adjacent longer sides.

[0023] In other words, in the context of this disclosure, a facet may be considered to be substantially kite-shaped if it has four main sides (a pair of relatively short main sides, i.e. 'minor' sides, and a pair of relatively long main sides, i.e. 'major' sides), and optionally one or more small / short sides between main sides, the small sides being significantly shorter than the main sides (such as e.g. below about 10%, preferably below about 5% of the length of the adjoining main sides).

[0024] For example, in any aspects and embodiments of the invention the facets in the second set of crown facets may comprise an additional short side between the long, major sides. The short side is preferably significantly shorter than the adjoining major (long) sides L150, and may for example have a length of between 0 and about 10%, between 0 and about 5%, or between 0 and about 2% of the length of the major sides L150. Similarly, in some embodiments, the facets in the second set of crown facets may comprise an additional short side between the short, minor sides. The additional short side is preferably significantly shorter than the adjoining minor sides S150, and may for example have a length of between 0 and about 10%, between 0 and about 5%, or between 0 and about 2% of the length of the minor sides S150.

[0025] In embodiments of any aspect, one or more of the facets in the third set of crown facets may comprise an additional short side between the short / minor sides. This may occur, for example, due to small errors in the location of the facets of the first and second sets of crown facets such that some facets in the first set of crown facets do not adjoin the facets of the third set of crown facets at a perfect triangular tip. As such, the facets of the third set of crown facets may be pentagonal or hexagonal, with four main sides (comprising two major sides and two minor sides) and one or two small / short sides between pairs of main sides.

[0026] In embodiments of the second aspect, the facets in the third set of main crown facets are substantially kite-shaped. Typically, the facets of the third set of main crown facets adjoin the facets of the first set of main crown facets at their long sides.

[0027] In embodiments of the second aspect, the facets in the second set of crown facets and the facets in the third set of main crown facets are substantially kite-shaped. Typically, the facets in the second set of crown facets adjoin the facets of the third set of main crown facets with their respective short sides.

[0028] In embodiments of the second aspect, the facets in the first set of main crown facets are substantially triangular. In some embodiments of the first aspect, the facets in the first set of main crown facets are substantially pentagonal. In embodiments, the side of the triangle / pentagon that adjoins the girdle may be curved. As discussed with respect to other facets above, in some embodiments, one or more of the facets in the first set of main crown facets is / are triangular / pentagonal with a small flat tip at the apex of the triangle or pentagon that is furthest from the girdle. In other words, an additional small / short side may be present between the main sides of the facets that extend away from the girdle, where the additional small side has preferably at most about 10%, at most about 5% or at most about 2% of the length of the adjoining main sides of the facet.

[0029] In embodiments of the second aspect, the first and third sets of main crown facets suitably alternate around the perimeter of the crown such that the number of facets in the first set of main crown facets is equal to the number of facets in the third set of main crown facets. The number of facets in the second set of crown facets may be equal to the number of facets in the first or third set of main crown facets. Similarly, in embodiments of the first aspect, the number of facets in the second set of crown facets may be equal to the number of facets in the first set of main crown facets.

[0030] In embodiments of the second aspect, the facets in the second set of crown facets are arranged at azimuthal angles that are the arithmetic average of the azimuthal angles at which adjoining facets in the third set of main crown facets are arranged. In embodiments of the first aspect, the facets in the second set of crown facets are arranged at azimuthal angles that are the arithmetic average of the azimuthal angles at which adjoining facets in the first set of main crown facets are arranged.

[0031] In embodiments of any aspect, the number of facets in the first or third (where present) set of main crown facets is 6, 7, 8 or 9. In some embodiments, the number of facets in the second set of crown facets is equal to 6, 7, 8 or 9; or may be 12, 14, 16 or 18. In embodiments, the crown facets of the gemstone have 6, 7, 8 or 9-fold symmetry. In some embodiments, the crown facets have 7-fold symmetry. Suitably, the number of facets in each set of the first and third set of main crown facets is 7. Advantageously, the use of 7-fold symmetry / 7 facets in the first and third sets of main crown facets has been shown to result in particularly high fire values. In such embodiments the number of facets in the second set of crown facets may be 7 or 14; preferably 7.

[0032] In embodiments of any aspect, all of the facets of the crown belong to the first set of main crown facets, the second set of crown facets and the third set of main crown facets (if present).

[0033] In embodiments of the second aspect, the angle between the facets of the second set of crown facets and the plane of the girdle is smaller than both (i) the angle between the facets of the first set of main crown facets and the plane of the girdle, and (ii) the angle between the facets of the third set of (main) crown facets and the plane of the girdle. In embodiments of the first aspect, the angle between the facets of the second set of crown facets and the plane of the girdle is smaller than the angle between the facets of the first set of main crown facets and the plane of the girdle. The use of a smaller (shallower) angle for the facets in the second set of crown facets also advantageously reduces the risks of damage to the apex of the crown and improves the ease of manipulation and application of the gemstone onto surfaces such as e.g. the surface of articles onto which the gemstones are to be applied.

[0034] In embodiments of the second aspect, the angle between the facets of the first type of main crown facets and the plane of the girdle is larger than the angle between the facets of the third set of (main) crown facets and the plane of the girdle.

[0035] In embodiments of any aspect, the angle between the facets of the first set of main crown facets and the plane of the girdle may be at most about 43°. In embodiments of any aspect, the angle between the facets of the first set of main crown facets and the plane of the girdle may be at least 38°. Thus, in some embodiments of any aspect, the angle between the facets of the first set of main crown facets and the plane of the girdle may be in the range of between about 38° and about 43°.

[0036] In embodiments of the first aspect, the angle between the facets of the first set of main crown facets and the plane of the girdle is in the range of 40.4° ± 6°, 40.4° ± 3°, 40.4° ± 1.5° or 40.4° ± 0.75°.

[0037] In embodiments of the second aspect, the angle between the facets of the first set of main crown facets and the plane of the girdle is in the range of 42.20° ± 6°, 42.20° ± 3°, 42.20° ± 1.5° or 42.20° ± 0.75°.

[0038] In embodiments of any aspect, the angle between the facets of the second set of crown facets and the plane of the girdle may be at most about 26°. In embodiments, the angle between the facets of the second set of crown facets and the plane of the girdle may be at least 12°. Thus, in some embodiments of any aspect, the angle between the facets of the second set of main crown facets and the plane of the girdle may be in the range of between about 12° and about 26° or between about 16° and about 21°.

[0039] In embodiments of the first aspect, the angle between the facets of the second set of crown facets and the plane of the girdle is in the range of 18.2° ± 6°, 18.2° ± 3°, 18.2° ± 1.5°, or 18.2° ± 0.75°.

[0040] In embodiments of the second aspect, the angle between the facets of the second set of crown facets and the plane of the girdle is in the range of 19.2° ± 6°, 19.2° ± 3°, 19.2° ± 1.5°, or 19.2° ± 0.75°. In some particular embodiments of the second aspect, the angle between the facets of the second set of crown facets and the plane of the girdle is in the range of 19.16° ± 0.75°.

[0041] In embodiments of the second aspect, the angle between the facets of the third set of (main) crown facets and the plane of the girdle is in the range of 36.1° ± 6°; or in the range of 36.1° ± 3°; or in the range of 36.1° ± 1.5°; or in the range of 36.1° ± 0.75°.

[0042] In some preferred embodiments of the second aspect, the angle between the facets of the first set of main crown facets and the plane of the girdle is between about 38° and about 43°, the angle between the facets of the third set of (main) crown facets and the plane of the girdle is between about 33° and about 38°, and the angle between the facets of the second set of crown facets and the plane of the girdle is between about 12° and about 26°. Advantageously, in embodiments, the angle between the facets of the first set of main crown facets and the plane of the girdle may be in the range 42.20° ± 0.75°, the angle between the facets of the third set of (main) crown facets and the plane of the girdle may be in the range 36.1° ± 0.75°, and the angle between the facets of the second set of crown facets and the plane of the girdle may be in the range 19.2° ± 0.75° or 19.16° ± 0.75°.

[0043] According to a third aspect of the invention, there is provided a method of improving the optical properties of a gemstone having a flat back, the method comprising cutting a gemstone to provide first set of main crown facets inclined relative to the plane of the girdle around the perimeter of the crown, and a second set of crown facets meeting at an apex of the crown and spaced from the girdle by the facets of the first set of main crown facets.

[0044] According to a fourth aspect of the invention, there is provided a method of improving the optical properties of a gemstone having a flat back, the method comprising cutting a gemstone to provide first and third sets of main crown facets inclined relative to the plane of the girdle and alternating around the perimeter of the crown, and a second set of crown facets meeting at an apex of the crown. Suitably, the facets of the second set of crown facets are spaced from the girdle by the facets of the first and third sets of main crown facets.

[0045] Embodiments of the third and fourth aspects of the invention may comprise any of the features of the first and second aspects, respectively. Similarly, any of the features of the gemstone, any coatings or layers applied thereon and described in relation to the first aspect apply equally to the gemstone of the second aspect.

[0046] According to a further aspect, the invention provides an article comprising a gemstone according to the invention. In embodiments, the article is a decorative article. Embodiments of this aspect may comprise a gemstone having any of the features of the gemstone of the first or second aspect.

[0047] For the avoidance of any doubt, embodiments of any of the aspects of the invention may comprise any of the features described in relation to any other aspect of the invention, unless such features are clearly not compatible. Furthermore, it is explicitly stated that any of the features of any embodiment of the invention - for example, the embodiments of the first and second aspects of the invention described above - are (where not obviously incompatible), intended and envisaged to be combined in any and all combinations; and all such combinations are hereby encompassed. Thus, by way of example, any one or more optional feature of the first set of crown facets are intended to be combined with any one or more optional features of the second set of crown facets and any one or more optional features of the third set of crown facets (where present); and any such resulting gemstone is considered to represent a gemstone according to the invention.

Brief Description of the Drawings



[0048] One or more embodiments of the invention will now be described, by way of example, with reference to the appended drawings, in which:

Figures 1A, 1B and 1C show schematic top (Figure 1A), bottom (Figure 1B) and side (Figure 1C) views of a brilliant cut gemstone according to the prior art;

Figures 2A schematically illustrates a framework to quantify the brilliance of a gemstone, exemplified using a brilliant cut gemstone as shown on Figures 1A to C, showing the pattern of light reflections towards the eye of a user looking at the gemstone from the top (table), originating from light in steep angle areas, intermediate angle areas and shallow angle areas relative to the plane of the girdle of the gemstone (Figure 2A); and the combined light reflections from shallow, intermediate and steep angle light is show in Figure 2B;

Figures 3A, 3B, 3C, 3D and 3E show schematic side (Figure 3A), top (Figure 3B) and top side perspective (Figure 3C) views of a first gemstone according to embodiments of the second aspect of the invention, and schematic side (Figure 3D) and top (Figure 3E) views of a second gemstone according to embodiments of the first aspect of the invention, both with a 7-fold symmetry of the crown facets;

Figures 4A, 4B, 4C and 4D show the pattern of light reflections associated with the gemstone of Figures 3A to 3C: Figure 4A shows the pattern of light reflections from shallow angle light; Figure 4B shows the pattern of light reflections from intermediate angle light; Figure 4C shows the pattern of light reflections from steep angle light; and Figure 4D shows the combined light reflections from shallow, intermediate and steep angle light;

Figures 5A, 5B, 5C and 5D show the pattern of light reflections associated with the gemstone of Figures 3D to 3E: Figure 5A shows the pattern of light reflections from shallow angle light; Figure 5B shows the pattern of light reflections from intermediate angle light; Figure 5C shows the pattern of light reflections from steep angle light; and Figure 5D shows the combined light reflections from shallow, intermediate and steep angle light;

Figures 6A, 6B and 6C show schematic side (Figure 6A), top (Figure 6B) and top side perspective (Figure 6C) views of a gemstone according to embodiments of the second aspect of the invention with a 6-fold symmetry of the crown facets;

Figures 7A, 7B and 7C show schematic side (Figure 7A), top (Figure 7B) and top side perspective (Figure 7C) views of a gemstone according to embodiments of the second aspect of the invention with an 8-fold symmetry of the crown facets;

Figures 8A, 8B and 8C show schematic side (Figure 8A), top (Figure 8B) and top side perspective (Figure 8C) views of a gemstone according to embodiments of the second aspect of the invention with a 9-fold symmetry of the crown facets;

Figures 9A and 9B show schematic side (Figure 9A), and top (Figure 9B) views of a gemstone according to embodiments of the first aspect of the invention with an 8-fold symmetry of the crown facets;

Figures 10A and 10B show schematic front side perspective views of flat back gemstones according to the prior art, with a simple cut (Figure 10A, A2000 cut) and an advanced cut (Figure 10B, A2078 cut); and Figure 10C shows the quantified fire value (relative to the maximum fire possible) and light return value (relative to the light return of Spectralon®) for the cuts of Figure 10A (A2000) and Figure 10B (A2078);

Figures 11A, 11B, 11C and 11D show the pattern of light reflections associated with the gemstone of Figure 10A: Figure 11A shows the pattern of light reflections from shallow angle light; Figure 11B shows the pattern of light reflections from intermediate angle light; Figure 11C shows the pattern of light reflections from steep angle light; and Figure 11D shows the combined light reflections from shallow, intermediate and steep angle light;

Figures 12A, 12B, 12C ,12D and 12E show the pattern of light reflections associated with the gemstone of Figure 10B, as well as the simulated illuminance along a section through the bright central area of the gemstone of Figure 10B: Figure 12A shows the pattern of light reflections from shallow angle light; Figure 12B shows the pattern of light reflections from intermediate angle light; Figure 12C shows the pattern of light reflections from steep angle light; Figure 12D shows the combined light reflections from shallow, intermediate and steep angle light; and Figure 12E shows the simulated illuminance along a section through the bright area of the gemstone of Figure 10B at the point and orientation indicated by a short white line in Figure 12D;

Figure 13 shows the quantified fire value (relative to the maximum fire possible) and light return value (relative to the light return of Spectralon®) for modified versions of the cuts of Figure 10A (A2000m) and Figure 10B (A2078m, which represents another embodiment of the invention), where both cuts are modified by elongating the crown facets to an apex (as illustrated directly below the respective points on the graph). Values of fire and light return for these modified cuts are shown in comparison to the gemstone of Figures 3A to 3C (marked as 'invented cut');

Figures 14A, 14B, 14C and 14D show the pattern of light reflections associated with the gemstone of the invention marked as 'A2078m' on Figure 13, i.e. a modified version of the gemstone of Figure 10B in which the crown facets have been extended to form an apex at the top of the gemstone: Figure 14A shows the pattern of light reflections from shallow angle light; Figure 14B shows the pattern of light reflections from intermediate angle light; Figure 14C shows the pattern of light reflections from steep angle light; and Figure 14D shows the combined light reflections from shallow, intermediate and steep angle light;

Figures 15A, 15B and 15C show an investigation of the luminance due to reflection of steep angle light through the bright area in the centre of the gemstone marked as 'A2078m' on Figure 13, i.e. a modified version of the gemstone of Figure 10B in which the crown facets have been extended to form an apex at the top of the gemstone: Figure 15A shows the pattern of reflections from steep angle light associated with this gemstone (identical to Figure 14C), with a short inclined white line and a long horizontal white line drawn through the central bright area; Figure 15B shows the simulated illuminance along the long (horizontal) line shown on Figure 15A; and Figure 15C shows the simulated illuminance along the short (inclined) line shown on Figure 15A;

Figures 16A and 16B show an investigation of the luminance due to reflection of steep angle light through the bright area in the centre of the gemstone according to the invention marked as 'invented cut' on Figure 13 and illustrated in Figures 3A to 3C: Figure 16A shows the pattern of reflections from steep angle light associated with this gemstone (identical to Figure 4C), with the section through the central bright area indicated by a white line; Figure 16B shows the simulated illuminance along the section shown on Figure 16A;

Figure 17 shows the quantified fire value (relative to the maximum fire possible) and light return value (relative to the light return of Spectralon®) for various embodiments of the second aspect of the invention, wherein the point labelled as 'invented cut' is a gemstone as shown on Figures 3A to 3C; the point labelled as 'invented cut, sym8' is a gemstone as shown on Figures 7A to 7C; the point labelled as 'invented cut, sym6' is a gemstone as shown on Figures 6A to 6C;

and the point labelled as 'invented cut, sym9' is a gemstone as shown on Figures 8A to 8C;

Figures 18A, 18B and 18C show the pattern of light reflections (combined light reflections from shallow, intermediate and steep angle light) associated with various gemstones according to the first aspect of the invention comprising first and second sets of crown facets: Figure 18B shows the pattern of combined light reflections from the gemstone of Figures 9A and 9B; Figure 18A and Figure 18C show the pattern of combined light reflections from gemstones according to the invention as described in Example 4 below.

Figures 19A and 19B show schematic top (Figure 19A), and side perspective (Figure 19B) views of a gemstone according to embodiments of the first aspect of the invention, with a 7-fold symmetry of first main crown facets and an 8-fold symmetry of second crown facets;

Figures 20A and 20B show schematic top (Figure 20A), and side perspective (Figure 20B) views of a gemstone according to embodiments of the first aspect of the invention, with an 8-fold symmetry of first main crown facets and a 9-fold symmetry of second crown facets;

Figure 21 shows the quantified fire value (relative to the maximum fire possible) and light return value (relative to the light return of Spectralon®) for modified gemstone embodiments according to the first aspect of the invention as illustrated in Figures 9A and 9B (marked as 'invented cut 2'); and Figures 3A to 3C (marked as 'invented cut').


Detailed Description



[0049] The present inventors have surprisingly discovered that a gemstone with superior optical properties could be obtained by providing either two or three sets of crown facets: a first set of main crown facets and a second set of crown facets that meet at an apex of the crown; in some aspect and embodiments comprising a third set of (main) crown facets that alternate around the perimeter of the crown with the first set of main crown facets. The second set of crown facets is spaced from the pavilion by the first and, in some embodiments, the third sets of main crown facets.

[0050] Throughout this description, the terms 'back' / 'bottom', and 'front' / 'top' surface are used to refer to the surfaces of a gemstone that when incorporated in an article (such as e.g. when applied to the surface of an article), are intended to face towards a viewer (front / top surface) or away from a viewer, such as towards the surface on which the gemstone is applied or supported (back / bottom surface), respectively. However, the skilled person will appreciate that gemstones may have a complex geometry, as required by the circumstances, and as such a back or front surface may, in fact, comprise a collection of jointed or disjointed surfaces. In practice, a front surface is intended to be visible in use, whereas a back surface is generally intended to be attached to or otherwise combined with an article.

[0051] Figures 1A, 1B and 1C show schematic top (Figure 1A), bottom (Figure 1B) and side (Figure 1C) views of a brilliant cut gemstone 1 according to the prior art. The gemstone 1 comprises a crown 2, a pavilion 6, and a girdle 4. The crown 2 forms the top (or 'front') part of the gemstone 1. In the case of a brilliant cut as illustrated on Figures 1A to 1C, the crown comprises a large flat facet 12 called the 'table' which is parallel to the plane of the girdle 4, and three types of crown facets 8, 10, 14 provided between the table 12 and the girdle 4. Facets 8 are commonly referred to as 'bezel facets'. Facets 10 are commonly referred to as 'star facets'. Facets 14 are commonly referred to as 'upper half facets'. The gemstone illustrated on Figures 1A to 1C has an 8-fold symmetry in both the crown 2 and the pavilion 6. As such, the crown comprises an octagonal table 12, eight star facets 10, eight bezel facets 8, and sixteen upper half facets 14. Pairs of upper half facets 14 alternate with single bezel facets 8 around the upper perimeter of the pavilion 6. The pavilion 6 forms the bottom (or 'back' or 'lower') part of the gemstone 1. In the case of a brilliant cut as illustrated in Figures 1A to 1C, the pavilion 6 comprises two types of facets. A first set of pavilion facets 16 is commonly referred to as 'pavilion main facets' and meet at an apex 20. The apex 20 may be an apex in the strict geometric sense of the word, or may be in the form of a small flat facet referred to as a 'culet'. The culet 20, when present, is typically significantly smaller than the other pavilion facets. A second set of pavilion facets 18 is commonly referred to as 'lower half facets'. As the gemstone illustrated on Figures 1A to 1C has an 8-fold symmetry in the pavilion 6, the pavilion 6 comprises eight pavilion main facets 16 and sixteen lower half facets 18. Pairs of lower half facets 18 alternate with single pavilion main facets 16 around the lower perimeter of the pavilion 6.

[0052] The girdle 4 is the region that forms the junction between the crown 2, and the pavilion 6, if present. As such, the girdle 4 is the region at which the gemstone has its largest transverse dimension. The girdle is associated with an imaginary plane PG, referred to as the 'plane of the girdle' or 'girdle plane', illustrated by the dashed line on Figure 1C. The plane PG is arranged such that all of the crown facets (and all of the pavilion facets, if present) within a set of facets have the same orientation and distance to the plane PG, when taking into account symmetries in the set. For example, the upper half facets 14 are provided in pairs where each pair comprises facets that mirror each other. Within the context of the invention, a 'set' or 'type' of facets refers to a group of facets that are substantially identical in shape, and that have the same or symmetrical orientations relative to the girdle plane PG.

[0053] In order to characterise the optical properties of a gemstone, a series of parameters which collectively define the 'brilliance' of a gemstone have been defined by the Gemological Institute of America in Moses et al., 2004 (Gems & Gemology, Fall 2004, Vol. 40, No. 3, https://www.gia.edu/gems-gemology/fall-2004-grading-cut-quality-brilliant-diamond-moses). These parameters are now standard in the art and include the fire, light return and scintillation. The fire of a gemstone refers to the extent of light dispersed into spectral colours seen in a polished gemstone when viewed face-up. The light return is the extent of internal and external reflections of 'white' light seen in a polished gemstone when viewed face up. The scintillation is the appearance of spots of light seen in a polished gemstone when viewed face up that flash as the gemstone, observer, or light source moves (also referred to as 'sparkle'); and the relative size, arrangement, and contrast of bright and dark areas seen in a polished gemstone when viewed face up while that gemstone is still or moving (also referred to as 'pattern'). These parameters can be quantified using an apparatus set up defined by the American Gemological Society in J. Sasian et al. (Evaluation of brilliance, fire, and scintillation in round brilliant gemstones, Optical Engineering, 46(9), 093604 (2007)) and P. Yantzer et al. (Foundation, Research Results and Application of the New AGS Cut Grading System, https://cdn.ymaws.com/www.americangemsociety.org/resource/resmgr/docs/AGSLab/A GS-Cut-System.pdf), as shown in Figure 2A which exemplifies the set up for a brilliant cut as shown on Figures 1A to C.

[0054] The system for measuring parameters of a gemstone as illustrated in Figure 2A provides a light source around a hemisphere H centred on the gemstone 1 and extending from the girdle plane PG of the gemstone 1 under evaluation, and evaluates the influence of light coming from different angles around the hemisphere H on light reflections seen by an observer O of the gemstone from the front / top centre of the gemstone (i.e. perpendicular to the girdle plane PG, through the table 12 of the gemstone if present). The hemisphere H is divided into three distinct segments 22, 24, 26, and the light reflections caused by illumination of these segments are separately evaluated; specifically, the hemisphere H is divided into a segment 26 of shallow-angle light, a segment 24 of intermediate angle light, and a segment of steep angle light 22. The pattern of reflections caused by light in each of these segments can be represented as separate shallow-angle, intermediate-angle and steep-angle representations 26', 24', 22' (illustrated in Figure 2A), respectively; and these can be combined into a single representation 28 (Figure 2B). Analysis of such patterns have indicated that an ideal brilliant cut as shown in Figures 1A to 1C should have about 15% of steep-angle light reflection (shown as relatively lighter areas in the pattern 22', relative to the complete surface area of the reflection pattern), which are evenly distributed and preferably display a star-like pattern; and where the steep-angle light reflection pattern does not comprise a large compact light area in the centre (i.e. no large relatively lighter area in the centre of pattern 22'). An ideal brilliant cut should also preferably have a high fraction of intermediate-angle light areas (shown as relatively lighter areas in the pattern 24'), and a small fraction of shallow-angle light areas (shown as relatively lighter areas in the pattern 26'). An 'Ideal Cut' (Tolkowsky) diamond gemstone with desirable properties (such as e.g. the brilliant cut of Figures 1A to 1C) may have light return and fire values of about 69% and 50%, respectively for, where fire can be quantified as a percentage of the maximum possible fire (all white light incident on the gemstone is reflected onto a measurement area and the colour saturation of this reflected light is complete); and light return can be quantified as a percentage compared to the light return that would be obtained from a sample of Spectralon® of the same size as the gemstone under evaluation.

[0055] Figures 3A, 3B, 3C, 3D and 3E show schematic side (Figure 3A), top (Figure 3B) and top side perspective (Figure 3C) views of a first gemstone 100 according to embodiments of the invention, and schematic side (Figure 3D) and top (Figure 3E) views of a second gemstone 101 according to embodiments of the invention, both with a seven-fold symmetry of the crown facets. The gemstone 100, 101 comprises a crown 120 and a girdle 140. In the embodiment shown in Figures 3A to 3C, the crown 120 comprises a first set of main crown facets 110 and a third set of (main) crown facets 130. Facets in the first 110 and third 130 sets of main crown facets alternate around the perimeter of the crown 120, and a second set of crown facets 150, which meet at an apex 200.

[0056] In the embodiment shown in Figures 3D to 3E, the crown includes a first set of main crown facets 110 and a second set of crown facets 150, which meet at an apex 200.

[0057] In the embodiments shown in Figures 3A to 3E, the apex 200 is an apex in the strict geometric sense. However, the apex 200 may instead comprise a small flat or slightly rounded region, similar to the culet of the pavilion of a known gemstone. When the apex 200 comprises a small flat or slightly rounded region, this is typically so small as to be negligible compared to the surface area of the gemstone when viewed from the top (see Figures 3B and 3E). For example, the small flat / rounded region of apex 200 may represent less than 5%, less than 4%, less than 3%, less than 2% or less than 1% of the surface area of the gemstone when viewed from the top.

[0058] As best seen in Figures 3C and 3D, the facets 110 in the first set of main crown facets adjoin the girdle 140 with a broad side B110. In the embodiment shown in Figures 3D and 3E, the facets 110 in the first set of main crown facets adjoin the facets 150 of the second set of crown facets with edges S110. In the embodiment shown in Figures 3A to 3C, the facets 130 in the third set of (main) crown facets adjoin the girdle 140 with a point, and the facets 150 in the second set of crown facets with edges S130. Also, as depicted in the embodiment shown in Figures 3A to 3C, the facets 110 in the first set of main crown facets are substantially triangular, the side adjoining the girdle B110 being curved and resulting in a gemstone giving a circular impression when seen from the top (see Figures 3B and 3C). In the embodiment shown in Figures 3D and 3E, the facets 110 in the first set of main crown facets are substantially pentagonal with a pair of long edges L110, a pair of short edges S110 (equivalent to edges S150 of the second set of crown facets), and a side B110 adjoining the girdle, the side adjoining the girdle B110 being curved and resulting in a gemstone giving a circular impression when seen from the top (see Figure 3E).

[0059] The side adjoining the girdle B110 may have a different shape, and for example, may be more or less curved (or even straight) depending at least on the geometry of the girdle 140. One or more of the facets in the first set of main crown facets may be triangular or pentagonal with a small flat tip at the apex of the triangle / pentagon that is furthest from the girdle. For example, this may occur due to slight errors / offsets in the location of the facets 130, such that the tip of a facet 110 does not form a perfect triangle / pentagon. In other words, an additional short / small side may be present between the facet main sides that extend away from the girdle. The additional short / small side is preferably at most about 10%, at most about 5% or at most about 2% of the length of the adjoining main sides of the facet. Of course, in any embodiments of the invention a deliberate design choice may be made to introduce one or more short / small sides in place of a point or tip of a facet.

[0060] In the embodiments shown in Figures 3A to 3E, the facets 150, 150' in the second set of crown facets are substantially kite-shaped. As the skilled person would understand, a kite shape can be considered to represent a quadrilateral whose four sides can be grouped into two pairs of equal-length sides (a pair of minor (short) sides and a pair of major (long) sides) that are adjacent to each other. As such, facets 150, 150' comprise a first pair of sides / edges S150 and a second pair of sides / edges L150, edges S150 being shorter than edges L150. The facets 150, 150' of the second set of crown facets adjoin each other with their long sides L150 and adjoin the facets 130 of the third set of main crown facets (as shown on Figures 3A and 3B) or the first set of main crown facets 110' (as shown on Figures 3D and 3E) with their short sides S150. As the skilled person would understand, in embodiments where the apex 200 comprises a small flat or slightly rounded region, similar to a pavilion culet as explained above, the facets 150, 150' may comprise an additional short / small side between the long sides L150. The short / small side is preferably significantly shorter than the adjoining major / long sides L150, and may for example have a length of between 0 and about 10%, between 0 and about 5%, or between 0 and about 2% of the length of the major / long sides L150. One or more of the facets 150, 150' may instead or in addition comprise an additional short / small side between the minor (short) sides S150, for example due to small errors in the location of the facets 110, 130 or 110' such that some facets 110, 110' do not adjoin a facet 150, 150' at a perfect vertex / triangular tip. As such, the facets 150, 150' may be pentagonal or hexagonal, with four main sides L150, S150 and one or two short / small sides between pairs of main sides. In other words, in the context of this disclosure, a facet may be considered to be substantially kite-shaped if it has four main sides (a pair of minor (short) main sides and a pair of major (long) main sides), and optionally one or more short / small sides between main sides, the short / small sides being significantly shorter than the main sides (such as e.g. below about 10%, preferably below about 5% of the length of the adjoining main sides). In the embodiment shown in Figures 3A to 3C, the facets 130 in the third set of (main) crown facets are also substantially kite-shaped, comprising a pair of minor (short) edges S130 and a pair of major (long) edges L130. The facets 130 of the third set of main crown facets adjoin the facets 110 of the first set of main crown facets with their long sides L130. The facets 130 of the third set of main crown facets adjoin the facets 150 of the second set of crown facets with their short sides S130.

[0061] In the embodiment shown in Figures 3A to 3C, the number of facets 150 in the second set of crown facets is equal to the number of facets in each of the first 110 and third 130 sets of main crown facets. As the skilled person would understand, the first 110 and third 130 sets of main crown facets alternate around the perimeter of the crown such that the number of facets in the first set 110 of main crown facets is also equal to the number of facets in the third set 130 of (main) crown facets. Similarly, in the embodiment shown in Figures 3D and 3E, the number of facets 150' in the second set of crown facets is equal to the number of facets in the first 110' set of main crown facets. In the embodiments shown, the crown 120 has seven-fold symmetry and comprises seven facets in each of the first and optionally third sets of main crown facets 110, 130, and in the second set of crown facets. In other embodiments, there may be twice the number of facets in the second set of crown facets as in either of the first and third sets of main crown facets.

[0062] Referring again to the gemstone of Figures 3A to 3C, in this embodiment, the facets 150 in the second set of crown facets are arranged at azimuthal angles (i.e. angles around the perimeter of the gemstone when seen from the top) that are the arithmetic average of the azimuthal angles at which adjoining facets 130 in the third set of main crown facets (in the embodiment of Figures 3A to 3C) or adjoining facets 110' in the first set of main crown facets (in the embodiment of Figures 3D to 3E). For example, in the embodiment shown in Figure 3A to 3C, the facets 130 in the third set of main crown facets are arranged at azimuthal angles of 0°, 51.43° (360°/7), 102.86°, 154.29°, 205.72°, 257.15°, and 308.58°. Similarly, in the embodiment shown in Figure 3D and 3E, the facets 110' in the first set of main crown facets are arranged at azimuthal angles of 0°, 51.43° (360°/7), 102.86°, 154.29°, 205.72°, 257.15°, and 308.58°. In the embodiments shown in Figures 3A to 3C, the facets 150 in the second set of main crown facets are arranged at azimuthal angles of (0°+51.43°)/2 = 25.715°, (51.43°+102.86°)/2 = 77.145°, (102.86°+154.29°)/2 = 128.575°, (154.29°+205.72°)/2 = 180°, (205.72°+257.15°)/2 = 231.435°, (257.15°+308.58°)/2 = 282.865°, and (308.58°+360°)/2 = 334.29°.

[0063] The gemstones 100, 101, 102 further comprise a flat back surface 160 in lieu of a pavilion. The flat back surface 160 is parallel to the girdle plane PG of the gemstone 100, 101, 102. In the embodiments shown in Figures 3A to 3E, the girdle has a height hg, which is the shortest distance between the flat back surface 160 and an edge of a crown facet - in this case the broad side B110 of the facets 110 in the first set of main crown facets. Advantageously, the height hg of the girdle may be adjusted depending on the circumstances and the size of the gemstone. For example, in any of the embodiments of the invention, the height hg of the girdle may be between 0 and about 3 mm; between 0 and about 2 mm, between 0 and about 1 mm, or between 0 and about 0.5 mm. The appropriate height hg of the girdle may of course vary depending on the size of the gemstone. For example, the girdle (when present) has a height of no more than about 33% of the height of the crown, no more than about 25% of the height of the crown, no more than about 20% of the height of the crown, or no more than about 15% of the height of the crown. In some embodiments, the girdle (when present) has a height hg of no more than about 15%, and preferably no more than about 10% of the diameter of the gemstone.

[0064] While the embodiments shown in Figures 3A to 3E have a flat back surface 160, in other embodiments, the back surface may instead be formed as a shallow pavilion. For example, the gemstone may comprise a pavilion that has a shorter depth / height than the height of the crown 120, such as no more than about half the height of the crown, e.g. no more than about 20% of the height of the crown, or no more than about 10% of the height of the crown. In such embodiments, the pavilion may have a configuration similar to that shown in Figures 1B and 1C, but with significantly shallower angles. In some embodiments, the pavilion (where present) has a symmetry adapted to match the symmetry of the crown (e.g. a seven-fold symmetry in the case of a gemstone 100 with a seven-fold symmetry in the crown); in other embodiments, however, the symmetry of the pavilion may be different to the symmetry of the crown; for example, the crown may have seven- or eight-fold symmetry, whereas the pavilion may have eight-fold or nine-fold symmetry, respectively. In some cases, such as the embodiments depicted in Figures 19 and 20, for example, the crown may not have perfect symmetry due to a difference in the number of first main crown facets compared to the number of second crown facets. In such cases, for convenience, the crown may be assumed to have a symmetry equal to the number of the first main crown facets.

[0065] In the embodiments shown in Figures 3A to 3E, the girdle is substantially circular - as best seen in Figures 3B and 3E. Circular gemstones are commonly used for their optically pleasing symmetry. However, other shapes are possible and envisaged within the scope of the invention, such as for example cushion shapes, oval shapes, etc.

[0066] Each of the sets of crown facets can be described by an angle (also referred to as an elevation angle or orientation), between the facets and the girdle plane PG. As best seen in Figure 3A, α1 is the angle between the facets 110 of the first set of main crown facets and the plane of the girdle PG, α3 is the angle between the facets 130 of the third set of main crown facets and the plane of the girdle PG, and α2 is the angle between the facets 150 of the second set of crown facets and the plane of the girdle PG. In the embodiment shown in Figures 3A to 3C, the angle α2 between the facets of the second set of crown facets and the plane of the girdle PG is smaller than both (i) the angle α1 between the facets 110 of the first set of main crown facets and the plane of the girdle PG, and (ii) the angle α3 between the facets of the third set of (main) crown facets 130 and the plane of the girdle PG. Similarly, in the embodiment shown in Figures 3D and 3E, the angle α2 between the facets of the second set of crown facets 150' and the plane of the girdle PG is smaller than the angle α1 between the facets 110' of the first set of main crown facets and the plane of the girdle PG.

[0067] In all embodiments, the use of a relatively shallow angle for the facets in the second set 150, 150', 150" of crown facets may advantageously reduce the risk of damage to the apex 200 of the crown (for example compared to a configuration that would simply extend the facets of the first main crown facets and optionally the third (main) crown facets to an apex), and may improve the ease of manipulation and application of the gemstone onto surfaces such as e.g. those of articles. In the embodiment shown in Figures 3A to 3C, the angle α1 between the facets 110 of the first type of main crown facets and the plane of the girdle PG may be larger than the angle α3 between the facets 130 of the third set of (main) crown facets and the plane of the girdle PG.

[0068] In particular, the embodiment shown in Figures 3A to 3C has angles α1 = 42.20°, α3 = 36.06° and α2 = 19.16°, and the embodiment shown in Figures 3D and 3E has angles α1 = 38.1° and α2 = 17.6°, although the invention is not limited to those angles. In particular, the angle α2 between the facets of the second set of crown facets 150 and the plane of the girdle PG may be at most about 26°, for example, between about 12° and about 26°, preferably between about 16° and about 21°. The inventors have discovered that angles α2 between the facets of the second set of crown facets 150 and the plane of the girdle PG in the range of 19.2° ± 0.75° are particularly advantageous in terms of brilliance in embodiments comprising a first and third set of (main) crown facets 110, 130. Similarly, the inventors have discovered that angles α2 between the facets of the second set of crown facets 150' and the plane of the girdle PG in the range of 18.2° ± 2° are particularly advantageous in terms of brilliance in embodiments having a first set of main crown facets 110' and a second set of crown facets 150' (without having a third set of (main) crown facets). Further, the angle α1 between the facets of the first set of main crown facets 110' and the plane of the girdle PG may be at most about 43°, for example, between about 38° and about 43°. In embodiments comprising a first and third set of main crown facets 110, 130, angles α1 between about 40.5° and about 43° may be particularly advantageous. In this regard, the inventors have in particular discovered that angles α1 between the facets of the first set of main crown facets 110 and the plane of the girdle PG in the range of 42.2° ± 0.75° are advantageous in terms of brilliance in embodiments comprising first and third sets of main crown facets 110, 130. Similarly, the inventors have discovered that angles α1 between the facets of the first set of crown facets 110' and the plane of the girdle PG in the range of 40.4° ± 1.5° are particularly advantageous in terms of brilliance in embodiments comprising a first set of main crown facets 110' and a second set of crown facets 150' (without a third set of main crown facets). The angle α3 between the facets of the third set of main crown facets and the plane of the girdle may be at most about 38°, for example between about 33° and about 38°, preferably between about 35.5° and about 38°. The inventors have in particular discovered that angles α3 between the facets of the third set of (main) crown facets 130 and the plane of the girdle PG in the range of 36.1° ± 0.75° are beneficial in terms of brilliance.

[0069] As best seen on Figures 3B and 3E, each of the facets 110, 130, 150; 110', 150' has a radial length l110, l130 (where present), l150 which is the longest length of the respective facet in the radial direction of the gemstone 100, 101 when viewed in plan from above. The radial length l110, l130, l150 is substantially identical for all of the facets in a set of facet types. The radial length l150 of the facets 150, 150' in the second set of crown facets is shorter than the radial length l110 of the facets 110, 110' in the first set of main crown facets, and shorter than the radial length l130 of the facets 130 in the third set of crown facets, if present. For example, the radial length l150 of the facets 150, 150' in the second set of crown facets is at most 60%, at most 50%, at most 40% or at most 30% of the radial length l110 of the facets 110, 110' in the first set of crown facets. In embodiments comprising a first and third set of main crown facets 110, 130, the radial length l110 of the facets 110 is suitably between 80% and 120% of the radial length l130 of the facets 130; for example, between 85% and 115%, between 90% and 100%, or between 95% and 105%. In some such embodiments the radial length l110 of the facets 110 are approx. the same as the radial length l130 of the facets 130, and the radial length l150 of the facets 150 is at most 60%, at most 50%, at most 40%, or at most 30% of the radial length l110 of the facets 110 and the radial length l130 of the facets 130.

[0070] Alternatively, it can be convenient to measure the radial length of each set of facets relative to the radius of the gemstone (when viewed in plan from above). In embodiments having first, second and third sets of crown facets, the radial length l150 of the facets 150, in the second set of crown facets is at most 45%, at most 40%, at most 35%, at most 30% or at most 25% of the radius of the gemstone. In such embodiments, the radial length l110 of the facets 110, in the first set of crown facets is at most 75%, at most 70%, at most 65%, at most 60%, or at most 55% of the radius of the gemstone. In such embodiments, the radial length l130 of the facets 130, in the third set of crown facets is at most 80%, at most 75%, at most 70%, at most 65%, or at most 60% of the radius of the gemstone.

[0071] In embodiments having first and second sets of crown facets, but lacking a third set of crown facets, the radial length l150 of the facets 150', in the second set of crown facets is at most 70%, at most 60% at most 50%, at most 40%, or at most 30% of the radius of the gemstone. In such embodiments, the radial length l110 of the facets 110, in the first set of crown facets is at most 80%, at most 75%, at most 70%, at most 65%, or at most 60% of the radius of the gemstone.

[0072] Figures 4A, 4B, 4C and 4D show the pattern of light reflections associated with the gemstone of Figures 3A to 3C, as simulated using the ray tracing software SPEOS from ANSYS, with a refractive index nD = 1.56. Figure 4A shows the pattern of light reflections from shallow angle light, Figure 4B shows the pattern of light reflections from intermediate angle light, Figure 4C shows the pattern of reflections from steep angle light, and Figure 4D shows the combined reflections from shallow, intermediate and steep angle light (see e.g. Figure 2A and 2B). The data indicates that the gemstone according to the embodiment shown in Figures 3A to 3C has about 13% steep-angle light areas (shown as comparatively lighter areas in Figure 4C), 76% intermediate-light areas (shown as comparatively lighter areas in Figure 4B) and 11% shallow-light areas (shown as comparatively lighter areas in Figure 4A). This is in line with an aim of the invention to achieve light interaction properties, such as brilliance, to emulate that of an ideal brilliant cut gemstone, which may exhibit about 15% of steep-angle light areas, a high fraction of intermediate-angle light areas, and a small fraction of shallow-angle light areas. Further, the steep-angle light areas are evenly distributed and display a star-like pattern, and the steep-angle light areas do not comprise a large compact light area in the centre (at least compared to flat back gemstones of the prior art, as will be demonstrated further below). The inventors have further found that the gemstone according to embodiments of the invention, such as that of Figures 3A to 3C, exhibits a high fire value (about 60%) and a high light return value (about 50% light return value relative to Spectralon®).

[0073] Figures 5A, 5B, 5C and 5D show the pattern of light reflections associated with the gemstone of Figures 3D and 3E, as simulated using the ray tracing software SPEOS from ANSYS, with a refractive index nD = 1.56. Figure 5A shows the pattern of light reflections from shallow angle light, Figure 5B shows the pattern of light reflections from intermediate angle light, Figure 5C shows the pattern of reflections from steep angle light, and Figure 5D shows the combined reflections from shallow, intermediate and steep angle light. The data indicates that the gemstone according to the embodiment shown in Figures 3D and 3E has about 5.3% steep-angle light areas (shown as comparatively lighter areas in Figure 5C), 88.9% intermediate-light areas (shown as comparatively lighter areas in Figure 5B) and 5.8% shallow-light areas (shown as comparatively lighter areas in Figure 5A). Again, this is in line with an aim of the invention to emulate the light interaction properties (e.g. brilliance) of an ideal brilliant cut gemstone, which exhibits about 15% of steep-angle light areas, a high fraction of intermediate-angle light areas, and a small fraction of shallow-angle light areas. Further, the steep-angle light areas are evenly distributed and display a star-like pattern, and the steep-angle light areas do not comprise a large compact light area in the centre when compared to flat back gemstones of the prior art, as will be demonstrated further below. The inventors have further found that the gemstone according to this embodiment has a high fire value (about 55%) and a high light return value (about 70.7% light return value relative to Spectralon®).

[0074] Suitably, gemstones according to the invention have a relative light return of at least 30% and a fire of at least 40%. Beneficially, the gemstone has a relative light return of at least 40% and a fire of at least 50%. Light return and fire may be measured according to the Gemological Institute of America (GIA) standard as set out in Moses et al., 2004 (Gems & Gemology, Fall 2004, Vol. 40, No. 3, https://www.gia.edu/gems-gemology/fall-2004-grading-cut-quality-brilliant-diamond-moses), as implemented in WO 2015/027252 A1, which is incorporated herein by reference. Light return and fire properties for simulated gemstones may be simulated as explained below, for example, using a ray tracing software such as SPEOS from ANSYS (https://www.ansys.com/products/optical/ansys-speos).

[0075] Figures 6A, 6B and 6C show schematic side (Figure 6A), top (Figure 6B) and top side perspective (Figure 6C) views of a gemstone 100 according to embodiments of the invention, with a 6-fold symmetry of the crown facets. In the embodiment shown in Figures 6A to 6C, the first set of main crown facets 110 has 6 substantially triangular facets with a curved broad side B110 adjoining the girdle 140, alternating around the perimeter of the girdle with 6 kite-shaped main crown facets 130 of a third set of crown facets. The crown 120 of the gemstone 100 further has 6 kite-shaped facets 150 in a second set of crown facets, which meet at an apex 200. The gemstone 100 further comprises a flat back surface 160 parallel to the girdle plane PG. The gemstone of this embodiment may have similar characteristics to those discussed above in relation to the gemstone of Figures 3A to 3C; for example, similar angles α1, α2 and α3.

[0076] Figures 7A, 7B and 7C show schematic side (Figure 7A), top (Figure 7B) and top side perspective (Figure 7C) views of a gemstone 100 according to embodiments of the invention, with an eight-fold symmetry of the crown facets. In the embodiment depicted, the first set of main crown facets 110 has 8 substantially triangular facets with a curved broad side B110 adjoining the girdle 140, alternating around the perimeter of the girdle with 8 kite-shaped main crown facets 130 of a third set of crown facets. The crown 120 of the gemstone 100 further has 8 kite-shaped facets 150 in a second set of crown facets, which meet at an apex 200. The gemstone 100 further comprises a flat back surface 160 parallel to the girdle plane PG. The gemstone of this embodiment may have similar characteristics to those discussed above in relation to the gemstone 100 of Figures 3A to 3C; for example, similar angles α1, α2 and α3.

[0077] Figures 8A, 8B and 8C show schematic side (Figure 8A), top (Figure 8B) and top side perspective (Figure 8C) views of a gemstone 100 according to an embodiment of the invention, with a nine-fold symmetry of the crown facets. In the embodiment shown in Figures 8A to 8C, the first set of main crown facets has 9 substantially triangular facets 110 with a curved broad side B110 adjoining the girdle 140, alternating around the perimeter of the girdle with 9 kite-shaped main crown facets 130 of a third set of crown facets. The crown 120 of the gemstone 100 further has 9 kite-shaped facets 150 in a second set, which meet at an apex 200. The gemstone 100 further comprises a flat back surface 160 parallel to the girdle plane PG. The gemstone of this embodiment may have similar characteristics to those discussed above in relation to gemstone 100 of Figures 3A to 3C; for example, similar angles α1, α2 and α3.

[0078] Figures 9A and 9B show schematic side (Figure 9A) and top (Figure 9B) views of a gemstone 101 according to an embodiment of the invention, with an eight-fold symmetry of the crown facets. In the embodiment shown in Figures 9A and 9B, the first set of main crown facets 110' has 8 substantially pentagonal facets with a curved broad side B110 adjoining the girdle 140, adjoining each other with long sides L110. The crown 120 of the gemstone 101 further has 8 kite-shaped facets 150' of a second set, which meet at an apex 200. The gemstone 101 further comprises a flat back surface 160 parallel to the girdle plane PG. The gemstone of this embodiment may have similar characteristics to those discussed above in relation to gemstone 101 of Figures 3D and 3E; for example, similar angles α1 and α2. In particular, in the embodiment shown, α1= 40.4° and α2=18.2°. Similarly to that discussed above in relation to gemstone 101 of Figures 3D and 3E, the gemstone 101 of Figures 9A and 9B has radial lengths l110 and l150 which may have similar characteristics to those described above. The gemstone 101 of Figures 9A and 9B with these parameters had a fire of about 52% and a light return relative to that of Spectralon® of 76.2%.

[0079] While embodiments with six, seven, eight and nine-fold symmetry have been described and are particularly advantageous, other symmetries are envisaged. In particular, the concept demonstrated in relation to gemstones 100 and 100' can be extended to lower symmetries such as e.g. five and especially higher symmetries, such as e.g. ten and above. However, the embodiments with six to nine-fold symmetry, and especially seven-fold symmetry have been found to be particularly advantageous in terms of brilliance properties and ease of manufacture.

[0080] In still other embodiments of the invention there may be a different number of facets in the first and third (where present) sets of main crown facets compared to the number of facets in the second set of facets wherein the number of facets in the second set of crown facets does not equal twice the number of facets as in either of the first and third (where present) sets of main crown facets. Suitably, there may be more facets in the second set of crown facets compared to first and optionally third sets of main crown facets. For example, gemstone embodiments of the invention may have 6 first main crown facets and optionally 6 third (main) crown facets and 7 second crown facets; or may have 7 first main crown facets and optionally 7 third (main) crown facets and 8 second crown facets; or may have 8 first main crown facets and optionally 8 third (main) crown facets and 9 second crown facets; or may have 9 first main crown facets and optionally 9 third (main) crown facets and 10 second crown facets. In still other embodiments there may be 6 first main crown facets and optionally 6 third (main) crown facets and 8 second crown facets; or there may be 7 first main crown facets and optionally 7 third (main) crown facets and 9 second crown facets; or there may be 8 first main crown facets and optionally 8 third (main) crown facets and 10 second crown facets; or there may be 9 first main crown facets and optionally 9 third (main) crown facets and 11 second crown facets.

[0081] Particular alternative embodiments of the gemstone of the invention are depicted schematically in Figures 19A and 19B and in Figures 20A and 20B. As illustrated, the gemstone 102 of Figures 19A and 19B has 7 first main crown facets 110" and 8 second crown facets 150"; whereas the gemstone 102 of Figures 20A and 20B has 8 first main crown facets 110" and 9 second crown facets 150". It will be appreciated that the gemstones of Figures 19 and 20 could equally be modified to include 7 or 8 third (main) crown facets respectively. Exceptionally, in embodiments wherein the number of facets of the second type is different to the number of facets of the first (and optional third) types, the facets of the second type may have different geometries to one another. For example, the crown facets of the second type may include both triangular and kite-shaped facets, wherein the kite-shaped facets may be irregular (i.e. not formed of two major sides of identical length and two minor sides of identical length). Further, the triangular facets may not be equilateral or isosceles triangles, but rather may be scalene triangles.

[0082] In embodiments, the refractive index of the material of the gemstone is at least about 1.45. Typically, the material of the gemstone has a refractive index of at least about 1.5 and not more than about 1.8, such as between about 1.55 and about 1.7. Physical properties like the refractive index influence the path of the light through a gemstone. As such, refractive indices within these ranges may further increase the brilliance of the gemstone. A particular refractive index for the gemstones of the invention is 1.56.

[0083] In embodiments, the gemstone is made of a transparent material. The term 'transparent' is used throughout this disclosure to refer to a material that has a transparency higher than zero. In the context of the present invention, a material is called transparent if it allows the transmittance of light, suitably at least visible light. Preferably, the material is transparent in the conventional sense, i.e. allowing (at least visible) light to pass through the material without being scattered.

[0084] The gemstone can be made of a wide variety of materials. In embodiments, the gemstone is made of glass, plastic or cubic zirconium. In some embodiments, the gemstone is made of crystal glass. Transparent bodies made of glass or plastic are preferred, because they are low cost, non-conductive and are most readily provided with facets. Gemstones made of glass, and in particular crystal glass (e.g. as defined by the European Crystal Directive (69/493/EEC)), can be particularly beneficial, for their superior optical properties.

[0085] The invention is not limited in principle with respect to the composition of the glass. 'Glass' in this context means any frozen supercooled liquid that forms an amorphous solid. Oxidic glasses, chalcogenide glasses, metallic glasses or non-metallic glasses can be employed. Oxynitride glasses may also be suitable. The glasses may be one-component (e.g. quartz glass) or two-component (e.g. alkali borate glass) or multicomponent (e.g. soda lime glass) glasses. The glass can be prepared by melting, by sol-gel processes, or by shock waves. Such methods are known to the skilled person. Inorganic glasses, especially oxidic glasses, are preferred. These include silicate glasses, soda lime glasses, borate glasses or phosphate glasses. Lead-free crystal glasses are particularly preferred.

[0086] In some embodiments, the gemstone is made of soda lime glass. Suitably, the faceted transparent body may alternatively be made of lead and barium-free crystal glass. Examples of suitable lead and barium-free crystal glass compositions for use in the present invention are disclosed in EP 1725502 and EP 2625149, the contents of which are incorporated herein by reference.

[0087] As another raw material for the preparation of the gemstone, plastics can be employed. Transparent plastics are preferred. Among others, the following materials are suitable: acrylic glass (polymethyl methacrylates, PMMA); polycarbonate (PC); polyvinyl chloride (PVC); polystyrene (PS); polyphenylene ether (PPO); polyethylene (PE); poly-N-methylmethacrylimide (PMMI).

[0088] An advantage of using a plastics material over glass in the manufacture of transparent bodies for use in the present invention resides, in particular, in the lower specific weight, which is only about half that of glass. In addition, other material properties may also be selectively adjusted. Further, plastics are often more readily processed as compared to glass. Some disadvantages of the use of plastics materials include the low modulus of elasticity and the low surface hardness as well as the massive drop in strength at temperatures from about 70°C and above, as compared to glass.

[0089] A preferred plastic is poly-N-methylmethacrylimide, which is sold, for example, by Evonik under the name Pleximid® TT70. Pleximid® TT70 has a refractive index of 1.54, and a transmittance of 91 % as measured according to ISO 13468-2 using D65 standard light.

[0090] In embodiments, the gemstone is coloured. In some such embodiments, the colouring is provided as a colouring agent throughout the material of the gemstone. Alternatively or in addition to colouring the body of the material, a colouring may be provided as a coating or surface treatment on at least a region of a surface of the gemstone. For example, a coating or surface treatment may be provided on at least a region of the crown facets and/or at least a region of a substantially flat back surface of the gemstone.

[0091] Colouring and decorative coatings may enable the gemstone to be provided with a variety of decorative effects, improving their flexibility of use. Colourings and decorative coatings are preferably configured such that the gemstone remains transparent to light entering through the crown facets.

[0092] In embodiments, the largest transverse dimension of the gemstone (also referred to herein as its diameter) may be between 1 and 80 mm, between 1 and 60 mm, between 1 and 40 mm, between 1 and 20 mm, or between 2 and 12 mm. In embodiments, the girdle may be substantially circular. In embodiments where the girdle is not substantially circular, the term 'diameter' may refer to the diameter of the smallest circle that would fit the geometry of the girdle.

[0093] In embodiments, a reflective layer is provided on substantially the whole of the back surface 160 of the gemstone. In embodiments, the reflective layer is a metallisation layer. In some such embodiments, the reflective layer comprises one or more metals selected from silver, aluminium and rhodium. Reflective layers such as metal coatings are known in the art.

[0094] In embodiments, the reflective layer is provided on a region of the back surface of the gemstone that covers most (such as e.g. 90%, 95%, 98%, or 99%) of the surface of the back surface of the gemstone.

[0095] In any aspect and/or embodiments of the invention, one or more layers of adhesive and/or one or more protective layers may be provided on substantially the whole of the back surface of the gemstone. The use of pre-applied adhesive may enable easy application of the finished decorative element to the surface of an article. A protective layer may advantageously protect the gemstone or a reflective layer applied thereon from damage, particularly before the gemstone is attached to a surface.

[0096] In any aspects and/or embodiments of the invention, a layer of non-reactive thermal adhesive may be provided on the back surface of the gemstone. Advantageously, a pre-applied hot-melt adhesive may enable easy application of the gemstone to many surfaces including e.g. garments and textiles, etc.

[0097] In embodiments, the layer of hot melt adhesive has a thickness between 100 µm and 200 µm. Advantageously, layers of hot melt adhesive in the above range may provide for sufficient adhesion even on porous substrates such as textiles. When the layer of hot melt adhesive is too thick, the risk of the hot melt adhesive spilling out when the decorative element is applied to a surface increases, resulting in possible application problems, loss of aesthetic quality, and waste of adhesive.

[0098] In embodiments, the hot melt glue may be a copolyamide-based glue, such as Griltex® 1A from EMS-CHEMIE. The hot melt glue may be a thermoplastic polyurethane-based glue, such as VP 1006 by Collano® AG.

[0099] In embodiments, the protective layer comprises a layer of lacquer. The lacquer may be applied with a thickness of between about 4 and 14 µm (i.e. 9 ± 5 µm); for example, the lacquer may be applied with a thickness of about 9 µm. In some embodiments, the layer of lacquer comprises a lacquer selected from the group consisting of: epoxy lacquers, one component polyurethane lacquers, bi-component polyurethane lacquers, acrylic lacquers, UV-curable lacquers, and sol-gel coatings. The lacquer may additionally ensure that the gemstone according to the invention is bondable. As the skilled person would understand, the choice of a suitable lacquer may depend on the material to which the gemstone is intended to be bonded, and/or on the adhesive that is intended to be used. The lacquer may optionally be pigmented. In embodiments, the lacquer is applied by spraying, digital printing, rolling, curtain coating or other two-dimensional application methods known in the art.

[0100] The gemstones according to the invention are particularly suitable as decorative elements for use on garments, wearables, fashion accessories, etc. where the combination of a flat back surface as well as the aesthetic potential afforded by the use of a gemstone with improved optical properties are important.

[0101] As such, the invention also encompasses an article comprising one or more gemstones according to the first and/or second aspect of the invention. For example, the article may be a clothing accessory such as shoes, a hat, sunglasses, glasses, bags, jewellery such as a bracelet, necklace or watch, an electronic wearable such as an activity tracker, etc., a piece of clothing such as a shirt, jacket, jumper etc., a consumer electronics item such as laptop, phone, tablet, etc.; a packaging article such as a box, can, jar, etc., a homeware article such as a frame, mirror, crockery item, etc. Articles also include vehicles, such as cars, boats etc.

[0102] Other variations of the invention will be apparent to the skilled person without departing from the scope of the appended claims.

Examples


Example 1



[0103] In this example, gemstone with flat backs according to the prior art were studied for their optical properties. In particular, gemstones 1000 and 1001 made of a material having a refractive index similar to that of crystal glass (i.e. nD = 1.56) and having a geometry as illustrated in Figures 10A and 10B with a mirror layer on their flat back were simulated using the ray tracing software SPEOS.

[0104] Figure 10A illustrates a schematic front side perspective view of a flat back gemstone 1001 according to the prior art, with a simple cut (referred to as 'A2000 cut') having a single set of crown facets 1100' extending between a table 1200 and a girdle 1400. Figure 10B shows a schematic front side perspective view of a flat back gemstone 1000 according to the prior art, with an advanced cut (referred to as 'A2078 cut') comprising a first and a third (according to the nomenclature used for embodiments of the invention described above) set of crown facets 1100, 1300 extending between a girdle 1400 and a flat table 1200. The table 1200 of gemstone 1000 had a shortest diameter of 1.55 mm and the table 1200 of gemstone 1001 has a shortest diameter of 1.15 mm. The gemstone of Figure 10A had a diameter of 3.1 mm, a girdle height of 0.31 mm and facets 1100' have an angle of 42.0° relative to the plane of the girdle. The gemstone of Figure 10B had a diameter of 3.1 mm, a girdle height of 0.18 mm, facets 1300 have an angle of 34.2° relative to the plane of the girdle, and facets 1100 have an angle of 40.7° relative to the plane of the girdle. In both cases the table facet 1200 has an angle of 0° to the girdle plane.

[0105] The fire and light return of these gemstones 1000, 1001 were simulated using the ray tracing software SPEOS, and as explained in Moses et al., 2004 (Gems & Gemology, Fall 2004, Vol. 40, No. 3, https://www.gia.edu/gems-gemology/fall-2004-grading-cut-quality-brilliant-diamond-moses), as implemented in WO 2015/027252 A1. In particular, for light return, an illumination arrangement around a hemisphere as illustrated in Figure 2A was used, such that diffuse light irradiates the crown of the gemstone. An observing section on the hemisphere centred on the gemstone and with a symmetrical aperture angle β of 2x1.5°, i.e. 3°, is used to determine the value of the reflected light return with regard to the incident light. This value was then normalised to the value that would be obtained with a corresponding sample of Spectralon®, expressed in %. For the measurement of fire, a directed white light source illuminating the gemstone from the top through an aperture of 2x0.25°, i.e. 0.5° was used. The coloured reflections on an observing surface (a 1 m x 1 m flat observing surface placed parallel to the girdle plane and at 0.5 m above the girdle plane), from the light incident on the gemstone were then analysed. In particular, the saturation and illuminance of the reflected light beams were quantified and the fire was quantified as Fire = 100 x ((∑(saturation x illuminance)) / (∑illuminance)) for each pixel of the observation surface.

[0106] The simulated fire and light return were plotted and the results displayed in Figure 10C, which shows the quantified fire value (relative to the maximum fire possible) and light return value (relative to the light return of Spectralon®) for the cuts of Figure 10A (A2000) and Figure 10B (A2078).

[0107] The pattern of light reflections associated with the gemstone of Figures 10A and 10B were also simulated for shallow, intermediate and steep-angle light, as explained above in relation to Figures 2A and 2B.

[0108] Figures 11A, 11B, 11C and 11D show the pattern of light reflections associated with the gemstone of Figure 10A. Figure 11A shows the pattern of light reflections from shallow angle light, Figure 11B shows the pattern of light reflections from intermediate angle light, Figure 11C shows the pattern of light reflections from steep angle light, and Figure 11D shows the combined light reflections from shallow, intermediate and steep angle light. These figures demonstrate that the flat back stone according to the prior art as shown in Figure 10A has poor distribution of light reflections from shallow, intermediate and steep angles. In particular, the gemstone 1001 has about 32% of steep angle light reflection areas (compared to an ideal of about 15%), with 64% intermediate and 4% of shallow light reflections. Further, the gemstone 1001 comprised a large compact steep-angle light area in the centre of the gemstone (see Figure 11C), and the steep-angle light areas were not distributed in a star-like pattern.

[0109] Figures 12A, 12B, 12C and 12D show the pattern of light reflections associated with the gemstone of Figure 10B. Figure 12A shows the pattern of light reflections from shallow angle light, Figure 12B shows the pattern of light reflections from intermediate angle light, Figure 12C shows the pattern of light reflections from steep angle light, and Figure 12D shows the combined light reflections from shallow, intermediate and steep angle light. These figures illustrate that the flat back gemstone according to the prior art as shown in Figure 10B has improved distribution of light reflections from shallow, intermediate and steep angles compared to the flat back gemstone as shown on Figure 10A, but these remain relatively poor. In particular, the gemstone 1000 has about 14% of steep angle light reflection areas (compared to an ideal of about 15%), with 80% intermediate and 6% of shallow light reflections. The gemstone 1000 comprised a smaller compact steep-angle light area in the centre of the gemstone (see Figure 12C) compared to gemstone 1001 of Figure 10A, but this was still relatively large (about 1.15-1.2 mm). Additionally, the steep-angle light areas were not distributed in a star-like pattern (see Figure 12C).

[0110] Figure 12E shows the simulated illuminance of the prior art gemstone of Figure 10B along a section through the bright area of the gemstone at the point and orientation indicated by a short white line in Figure 12D, demonstrating a large central area of light transmission / reflection through the centre of the gemstone.

[0111] Therefore, this data indicates that these flat back gemstones according to the prior art tend to have poor optical properties compared to ideal brilliant cut stones.

Example 2



[0112] In this example, the inventors sought to improve on the flat back gemstones of the prior art (Example 1), by providing new geometries: (i) according to an exemplary embodiment of the invention as shown in Figures 3A to 3C (referred to as 'invented cut' herein and in Figure 13); (ii) a geometry that represents a simple adaptation of the geometry of the prior art gemstone illustrated in Figure 10A, wherein the crown facets 1100' are extended towards the middle of the gemstone to form an apex (referred to as 'A2000m' herein and in Figure 13); and (iii) a geometry according to the invention similar to that shown in Figures 3D and 3E, wherein the second set of crown facets are transformed to elongated kite-shaped facets that extend over half way from the apex of the gemstone towards the girdle of the gemstone (referred to as 'A2078m' herein and in Figure 13). The fire and light return values were quantified as explained above in relation to Example 1. Figure 13 shows the results of these analyses. As can be seen, the gemstones according to the invention ('invented cut' and 'A2078m') have significantly higher fire values than the simplistic modified gemstone A2000m. The gemstone of the invention with three sets of crown facets ('invented cut') has better fire than the gemstone 'A2078m', whereas the gemstone 'A2078m' has better light return when compared to the gemstone 'invented cut' - both embodiments of the invention also exhibit better light return than at least the A2000m gemstone.

[0113] Figure 21 shows the quantified fire value (relative to the maximum fire possible) and light return value (relative to the light return of Spectralon®) for embodiments according to the first aspect of the invention as illustrated in Figures 9A and 9B (marked as 'invented cut 2') and Figures 3A to 3C (marked as 'invented cut') in comparison to the embodiment termed 'A2078m' and the comparative gemstone cut termed A2000m both of which are described above. This data shows that the embodiment of 'invented cut' exhibits higher fire in comparison to both 'invented cut 2' and the gemstone embodiment 'A2078m'; whereas the embodiment of 'invented cut 2' beneficially exhibits higher light return relative to Spectralon® in comparison to both 'invented cut' and 'A2078m', and exhibits higher fire than 'A2078m'.

[0114] Figures 14A, 14B, 14C and 14D show the pattern of light reflections associated with the gemstone of the invention termed 'A2078m' above and in Figure 13. Figure 14A shows the pattern of light reflections from shallow angle light, Figure 14B shows the pattern of light reflections from intermediate angle light, Figure 14C shows the pattern of light reflections from steep angle light, and Figure 14D shows the combined light reflections from shallow, intermediate and steep angle light. These figures show that the modified version of the flat back gemstone of Figure 3D and 3E has improved distributions of light reflections from shallow, intermediate and steep angles compared to the closest prior art flat back gemstone as shown in Figure 10B (see Figures 12A to 12D).

[0115] In this regard, the gemstone A2078m has about 7% of steep angle light reflection areas (compared to an ideal of about 15%). The gemstone A2078m has 84% of intermediate angle light reflection areas and about 9% of steep angle light reflection areas (which is broadly in line with the ideal high proportion of intermediate angle light reflection areas and small proportion of shallow angle light reflection areas exhibited by a brilliant cut gemstone); and is better than the corresponding gemstone of the prior art as shown in Figure 10B. The gemstone A2078m still comprises an area of steep-angle light reflection in the centre of the gemstone (see Figure 14C), but this is far more compact than in the corresponding prior art gemstone of Figure 10B.

[0116] The characteristics of this compact steep-angle light area in the centre of the gemstone A2078m (as shown in Figure 14C) were then investigated in more detail. Figures 15A, 15B and 15C show an investigation of the luminance due to reflection of steep angle light through the bright area in the centre of the gemstone marked as 'A2078m" on Figure 13. Figure 15A shows the pattern of reflections from steep angle light associated with this gemstone (identical to Figure 14C) along the short line (angled from upper left to lower right) and the long line (approx. horizontal) through the central bright area indicated by white lines in Figure 15A. Figure 15B shows the simulated illuminance along the long, horizontal line in Figure 15A, and Figure 15C shows the simulated illuminance along the short, inclined line shown in Figure 15A. The data in Figures 15B and 15C show that the gemstone A2078m has a bright central area of about 1.1 mm along the long axis and about 0.6 mm along the short axis.

[0117] The same analysis was repeated for a gemstone according to embodiments of the invention with three sets of crown facets, which gemstone was selected to have the same diameter as gemstone A2078m (about 3.1 mm), although the results were expected to be independent of the gemstone diameter. Figures 16A and 16B show an investigation of the luminance due to reflection of steep angle light through the bright area in the centre of a gemstone according to the invention with three sets of crown facets, marked as "invented cut" on Figures 13 and 21 and illustrated in Figures 3A to 3C. Figure 16A shows the pattern of reflections from steep angle light associated with this gemstone (identical to Figure 4C), with the section through the central bright area indicated by a white line. Figure 16B shows the simulated illuminance along the section indicated by the white line shown in Figure 16A. This data indicates that the gemstone according to embodiments of the invention with three sets of crown facets has a central bright area of 0.64 mm, which is about 50% less than for A2078. Therefore, this data shows that the gemstone of the invention provides improved fire properties as well as the pattern of light reflections from at least steep angle areas compared to flat back gemstones according to the prior art.

Example 3



[0118] In this example, the inventors sought to demonstrate that the beneficial results evidenced above for gemstones of the invention having seven-fold symmetry can be mirrored with gemstones according to embodiments of the invention having eight or nine-fold symmetry. Figure 17 shows the quantified fire value (relative to the maximum fire possible) and light return value (relative to the light return of Spectralon®) for various embodiments of the invention. In the graph of Figure 17, the point labelled 'invented cut' shows the results for a gemstone of the invention as shown on Figures 3A to 3C having seven-fold symmetry; the point labelled 'invented cut, sym8' shows the results for a gemstone of the invention as shown in Figures 7A to 7C having eight-fold symmetry; the point labelled 'invented cut, sym6' shows the results for a gemstone of the invention as shown in Figures 6A to 6C having six-fold symmetry; and the point labelled 'invention cut, sym9' shows the results for a gemstone of the invention as shown in Figures 8A to 8C having nine-fold symmetry. This data shows that similarly high fire and light return values can be obtained with six-, eight- and nine-fold symmetries according to embodiments of the invention.

Example 4



[0119] In this example, the inventors sought to show that the beneficial results demonstrated above with gemstones of the invention having three sets of crown facets can be replicated with gemstones according to embodiments of the invention having just a first and a second set of crown facets. The following gemstones were simulated and their fire value (relative to the maximum fire possible) and light return value (relative to the light return of Spectralon®) were quantified:
  • a gemstone as shown on Figures 3D and 3E: 7-fold symmetry, α1=38.1°, α2=17.6°, I110=69% of the radius, I150=39% of the radius: this gemstone had a fire of 54.8% and a light return of 70.7%; the combined reflections from shallow, intermediate and steep angle light is shown in Figure 5D;
  • a gemstone similar to that shown on Figures 3D and 3E with 7-fold symmetry (7 facets 110' and 7 facets 150'), but where the radial length I150 of the facets in the second set of crown facets is longer than in the embodiment shown in Figures 3D and 3E, α1=39.2°, α2=19.4°, I110=56% of the radius, I150=56% of the radius: this gemstone had a fire of 55.6% and a light return of 65.1%; the combined reflections from shallow, intermediate and steep angle light is shown in Figure 18A;
  • a gemstone as shown on Figures 9A and 9B: 8-fold symmetry, α1=40.4°, α2=18.2°, I110=68% of the radius, I150=38% of the radius: this gemstone had a fire of 57.8% and a light return of 76.3%; the combined reflections from shallow, intermediate and steep angle light is shown in Figure 18B; and
  • a gemstone similar to that shown on Figures 9A and 9B with 8-fold symmetry (8 facets 110' and 8 facets 150'), but where the radial length I150 of the facets in the second set of crown facets is shorter than in the embodiment shown in Figures 9A and 9B, α1=40.6°, α2=16.2°, I110=76% of the radius, I150=28% of the radius: this gemstone had a fire of 56.7% and a light return of 60.2%; the combined reflections from shallow, intermediate and steep angle light is shown in Figure 18C.


[0120] This data show that similarly high fire and light return values can be obtained with gemstones having two sets of crown facets with seven and eight-fold symmetries, and with different ratios of radial lengths according to various embodiments of the invention.

Example 5



[0121] In this example, the inventors sought to demonstrate that the beneficial results evidenced above for gemstones of the invention having six, seven, eight or nine-fold symmetry can be mirrored with gemstones according to embodiments of the invention having non-perfect symmetry.

[0122] Accordingly, the gemstones according to the invention depicted in Figures 19A and 19B, and 20A and 20B were tested to measure the optical properties of fire and light return as described in Example 1. Beneficially, these gemstones were found to exhibit relatively high maximum values of fire and light return relative to Spectralon®.


Claims

1. A gemstone comprising a crown and a girdle at which the gemstone has its largest transverse dimension, the crown comprising a first set of main crown facets inclined relative to the plane of the girdle and a second set of crown facets meeting at an apex of the crown; wherein the gemstone optionally comprises a third set of crown facets inclined relative to the girdle and alternating with the facets of the first set of crown facets around the perimeter of the crown.
 
2. The gemstone of Claim 1, wherein the gemstone comprises a substantially flat back, preferably wherein a reflective layer is provided on the substantially flat back to reflect light incident on the reflective layer through the facets of the crown.
 
3. The gemstone of Claim 1 or Claim 2, wherein the facets in the second set of crown facets adjoin the facets of the first set of main crown facets, or the third set of crown facets if present.
 
4. The gemstone of any preceding claim, wherein the number of facets in the second set of crown facets is equal to the number of facets in the first set of main crown facets; preferably wherein the number of facets in the first set of main crown equals the number of facets in the third set of crown facets where present.
 
5. The gemstone of any preceding claim, wherein: (i) the number of facets in each of the first and second sets of crown facets is 6, 7, 8 or 9; (ii) the number of facets in each of the first and second sets of crown facets is 6, 7, 8 or 9 and wherein the number of facets in the first set of crown facets does not equal the number of facets in the second set of crown facets, preferably wherein the number of facets in the second set of crown facets is greater than the number of facets in the first set of crown facets; and/or (iii) wherein the number of facets in the first set of main crown facets is equal to the number of facets in the third set of crown facets, where present.
 
6. The gemstone of any preceding claim, wherein all of the facets of the crown belong to: (i) the first set of main crown facets and the second set of crown facets; or (ii) the first set of main crown facets, the second set of crown facets and the third set of crown facets.
 
7. The gemstone of any preceding claim, wherein the angle between the facets of the second set of crown facets and the plane of the girdle is smaller than both: (i) the angle between the facets of the first set of main crown facets and the plane of the girdle, and (ii) the angle between the facets of the third set of crown facets and the plane of the girdle where present.
 
8. The gemstone of any preceding claim, wherein the angle between the facets of the second set of crown facets and the plane of the girdle is at most about 26°; preferably wherein the angle between the facets of the second set of crown facets and the plane of the girdle is at least 12°.
 
9. The gemstone of Claim 8, wherein the angle between the facets of the second set of crown facets and the plane of the girdle is in the range of 19.2° ± 6° or about 19.2° ± 3°; preferably wherein the gemstone comprises the third set of crown facets and the angle between the facets of the second set of crown facets and the plane of the girdle is in the range of 19.2° ± 1.5°; or wherein the gemstone does not comprise the third set of crown facets and the angle between the facets of the second set of crown facets and the plane of the girdle is in the range of 18.2° ± 2°.
 
10. The gemstone of any preceding claim, wherein the angle between the facets of the first set of main crown facets and the plane of the girdle is at most about 48.2°; preferably wherein the angle between the facets of the first set of main crown facets and the plane of the girdle is at least 36.2°.
 
11. The gemstone of Claim 10, wherein the angle between the facets of the first set of main crown facets and the plane of the girdle is in the range of 42.20° ± 6°, or 42.20° ± 3°; preferably wherein the gemstone comprises the third set of crown facets and the angle between the facets of the first set of main crown facets and the plane of the girdle is in the range of 42.20° ± 1.5° or 42.20° ± 0.75°; or wherein the gemstone does not comprise the third set of crown facets and the angle between the facets of the first set of main crown facets and the plane of the girdle is in the range of 40.4° ± 1.5°.
 
12. The gemstone of any preceding claim, wherein the gemstone comprises the third set of crown facets and the angle between the facets of the third set of crown facets and the plane of the girdle is at most about 43°; preferably wherein the angle between the facets of the third set of crown facets and the plane of the girdle is at least 30°.
 
13. The gemstone of Claim 12, wherein the angle between the facets of the third set of crown facets and the plane of the girdle is in the range of 36.1° ± 6° or 36.1° ± 3°; preferably wherein the angle between the facets of the third set of main crown facets and the plane of the girdle is in the range of 36.1° ± 1.5° or 36.1° ± 0.75°.
 
14. An article, comprising a gemstone according to any of the preceding claims.
 
15. A method for improving the optical properties of a gemstone with a flat back, the method comprising cutting a gemstone to provide a first set of main crown facets inclined relative to the plane of the girdle and a second set of crown facets meeting at an apex of the crown; optionally wherein the method comprises providing a third set of crown facets alternating with the facets of the first set of crown facets around the perimeter of the crown.
 




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Cited references

REFERENCES CITED IN THE DESCRIPTION



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Patent documents cited in the description




Non-patent literature cited in the description