The present invention relates to a cut product manufactured from a (rough) natural or synthetic (semi) precious stone material, more particularly from natural or synthetic diamond, comprising:

- a lower part (pavilion) with a bottom end (culet) ;

- an upper part (crown) with a number of facets and a top end (a point with table width 0 or a top surface (table) with a table width) ; and

- a girdle between lower part and upper part .

Note that, in the case of synthetic diamond, the starting material (substrate) can be manufactured by any technical process, such as for instance chemical vapour deposition (CVD) or an HTHP process (high pressure high temperature process) . An example of a CVD process is the process used by Apollo Diamonds Inc .

Prior art

A cut of a precious stone is defined by its facets. These facets have a specific shape, a specific location and a specific angular position relative to each other. The combination of the specific shape, location and angular position of the different facets defines the cut. The angular position of the different facets of a cut are defined by the proportion parameters of this cut.

The value of a cut diamond is determined to a significant extent by the four Cs: Cut, Clarity, Carat and Colour. These are respectively the cut, the clarity, the weight and the colour of a cut diamond.

λCut' is understood to mean the finish of the stone. The shape in which the stone is cut forms part of this. The finish relates to the quality of the cutting and the ratios of the cut form. Figure IA illustrates schematically a prior art stone. The essence lies in the correct ratios and the refinement of the cut stone 20. The ratios are understood to mean the height 34 of crown 24, the crown angle β, the depth 35 of pavilion 25, the table reflection, the ratio of the girdle 22 relative to the total depth of the stone. Refinement is understood to mean the precise overall finish. How regular is the girdle, is the culet heavy or light, are there differences in symmetry between crown 24 and pavilion 25, do the facets connect precisely to each other, is the culet exactly in the centre or is the table off-centre? All these factors have a direct influence on the play of light in the stone. In contrast to the clarity, colour and partially also the weight, the cut is man-made. It is therefore a large price-determining factor in the four Cs, since a stone with a good round weight, which is flawless and has the best colour in the form of a brilliant cut seems to be a top stone. However, if the stone has been cut too deep (nail) or too shallow (fish-eye) , the play of light in the stone is dead, and the stone has a lower value. In short, the finish is of great importance because it eventually brings out the most important aspect of the stone: the brilliance and the play of colour in all its resplendence.

^λ Clarity' is the clarity of brightness of cut diamond. The stone can have both internal and external flaws. The internal flaws usually consist of carbon residues which have not fully crystallized, or glets (internal fissures) . They occur in many different forms but also in various grades of intensity. Growth lines which show the structure of the rough stone. There are also external characteristics, such as bearding which is left when the stone is cut too hard. It is also possible that natural remains when the stone is cut too little. Both characteristics can be seen on the girdle. All these characteristics determine the clarity of the stone, which is divided into different categories: Flawless, WSl, WS2, VSl, VS2, SI, Pique 1, Pique 2, Pique 3. The assessment hereof is carried out by the trained eye of the diamond merchant or in laboratories, under the microscope.

The weight of precious stones (Carat) is expressed in Carat (1 carat = 0.2 gram) . The carat is sub-divided into 100 points and is always expressed to two decimals, for instance 0.24 carat or 24 points .

Colour is always subjective. The whiter the colour, the higher the price. The colour is determined on the basis of a set of so-called master stones. This is a set of stones assessed by various leading diamond merchants and having different colours in the highest grades, which are deemed as a standard. The assessment usually takes place by eye. Electronic assessments are also possible nowadays.

The valuation of cut diamonds is however more complex in practice than measuring or determining the 4 Cs. In addition to the combination of the different Cs, the assessment of the colour, the clarity and the cut is often subjective. The finish and the life of a stone are moreover also taken into account. The life of a stone is a subjective measure of the brilliance of a diamond. The value of a cut diamond cannot therefore be fully determined on the basis of the 4 Cs. In this context reference is made to colour, brilliance, fire, scintillation and life as the optical characteristics.

Conventional cuts such as for instance brilliant, pear or oval have a specific shape because these specific shapes produce the ^λ most beautiful' cut diamonds. The assumption that these cuts produce the finest diamonds is based on the original work of Tolkowsky. Tolkowsky developed the modern brilliant in 1919. This brilliant, the Tolkowsky cut or Tolkowsky Brilliant, is the reference for living stones. A stone has life when it has beautiful brilliance and thus possesses optimal optical characteristics.

The optical characteristics of a cut diamond are explained by- various physical effects, of which the most important are the overall internal reflection and refraction of light on the surfaces. Since diamond has a very high refractive index (2.42), with a very small critical angle as a result, total reflection of the light will occur on many cut surfaces. The light will only leave the diamond at a small number of cut surfaces. In this context the cut has an extremely great influence on the refraction and the overall internal reflection. Figures 2A-C illustrate the path followed by the incident light. In the case of too great a pavilion height 35 - see figure IB - or too small a pavilion height - see figure 2C - light will escape, whereas an ideal cut - see figure 2A - results in a maximum internal reflection, whereby no light escapes and optimum optical characteristics are obtained.

It is generally accepted that Tolkowsky performed the first mathematical calculations which took into account the optical characteristics of a diamond. Tolkowsky calculated the 'ideal' proportion parameters of a brilliant. In other words, the proportion parameters are the degrees of freedom of a cut. Table 1 describes the limits of the different proportion parameters per category in respect of the assessment of the optical characteristics of conventional diamond cuts. Table 1

The cutting of new diamonds is thus limited by the above proportion parameters.

The invention

The present invention has for its object to provide a cut product of the type stated in the preamble, wherein the optical characteristics as defined above are better than for a cut product which is cut according to the existing indicated proportion parameters. For this purpose the invention is distinguished here in that the lower part comprises a number of girdle pavilion facets which describe an angle al relative to the plane of the girdle and a number of culet facets which each describe a smaller second angle a2 relative to the plane of the girdle,- wherein the average second angle a2 lies between 28 and 38 degrees; and wherein the ratio of the table size and the size of the girdle is 0 to 0.40.

Applicant has surprisingly found that by providing such facets and with a suitable choice of the smaller second angle and of the table dimensions, the light is reflected optimally in the stone, this resulting in improved optical properties. Note that the table dimensions are clearly smaller than what is usual according to conventional views (see the table above) and that angle a2 is smaller than is usual.

According to a preferred embodiment, the average second angle a2 lies between 25 degrees and 35 degrees, more preferably between 29 degrees and 33 degrees, and is for instance about 31 degrees. According to an even better variant, each second angle a2 lies between 28 degrees and 35 degrees, more preferably still between 29 degrees and 33 degrees, and is for instance about 31 degrees.

According to yet another aspect, the invention is distinguished in that the ratio of the table size and the size of the girdle is 0 to 0.3, and preferably either a point or a table with a table size amounting to between 10 and 30% of the size of the girdle.

The size of the table and the orientation of the facets of the upper and lower parts are further preferably optimized in order to reflect the lower part in at least a number of the facets of the upper part, and typically in the table bezel facets and/or the table and/or the girdle bezel facets .

The girdle pavilion facets are typically located above the culet facets, wherein each culet facet adjoins the bottom end and each girdle pavilion facet adjoins the girdle. According to a possible embodiment, the culet facets adjoin the girdle pavilion facets, although it is also possible for additional pavilions facet to be provided between the girdle and culet facets. Preferably at least three, and more preferably at least four, and for instance six, seven or eight girdle pavilion facets and culet facets are arranged.

According to yet another aspect of the invention, a number of bezels are provided between the top end and the girdle, wherein the number of bezels is equal to the number of pavilions (this is the number of girdle or culet facets) and the bezels are twisted relative to the pavilions through a twist angle γ. This is particularly useful when the number of bezels and pavilions is even. Note that each bezel can consist of a table bezel facet which adjoins the table and a girdle bezel facet which adjoins the girdle, wherein extra bezel facets can optionally also be included between the table bezel facets and the girdle bezel facets.

According to a variant in which this aspect is further developed, the twist angle is optimized as a function of the dimensions, and particularly as a function of the length, height and width of the stone, in order to obtain a reflection of the lower part in the bezels and/or in the table.

According to a possible embodiment, the average first angle al of the girdle pavilion facets lies between 15 and 80 degrees. Angles a2 and/or al can be further optimized for a cut product with the greatest possible volume. According to another aspect, the invention is distinguished in that the upper part is a crown with table bezel facets and girdle bezel facets, which girdle bezel facets describe a first angle βl relative to the plane of the girdle, and which table bezel facets describe a second, smaller angle β2 relative to the plane of the girdle. The average first angle βl of the girdle bezel facets relative to the plane of the girdle preferably lies between 35 degrees and 50 degrees, more preferably between 39 degrees and 43 degrees, and is most preferably about 41 degrees. The average second angle β2 preferably lies between 5 and 50 degrees, preferably between 30 and 50 degrees, and for instance between 31 and 41 degrees.

According to a preferred embodiment, the girdle bezel facets are located below the table bezel facets, wherein each table bezel facet adjoins the top end and each girdle bezel facet adjoins the girdle. The girdle bezel facets can adjoin the table bezel facets, although it is also possible to provide additional facets therebetween. Preferably at least three, more preferably at least four and for instance six, seven or eight girdle bezel facets and table bezel facets are arranged. The number of girdle and table bezel facets and the orientation thereof is preferably optimized for a cut product with the largest possible volume, taking into account of course all the parameters which are important for the value of the stone.

According to yet another aspect, the invention is distinguished in that the ratio of the height of the cut product and the width of the girdle lies between 0.60 and 1, and more preferably between 0.75 and 0.85. Another interesting parameter is the height of the stone at the edge of the table, and the ratio of this height relative to the table width. This is because this parameter will also play a part in obtaining an optimal reflection of the lower part in the upper part. According to yet another aspect, the invention is distinguished in that the ratio of the height of the girdle and the width of the girdle is 0.02 to 0.1. The girdle is preferably provided with a large number of facets in order to obtain a good reflection for light beams which are incident in the stone and reflected to the culet facets via the girdle.

According to the preferred embodiment of the invention, the lower part, and in particular the culet, is cut as a brilliant. Note that it is avantageous that the culet is cut as a brilliant because the culet side is reflected in the upper part .

The invention will be further elucidated on the basis of a number of non- limitative exemplary embodiments of the cut products according to the invention, with reference to the accompanying drawings .

Description of the figures

Figures IA-C illustrate the effects of the cut on the reflection and refraction of incident light;

Figure 2 is a schematic side view of an embodiment of a cut according to the invention, indicating the proportion parameters which are important in characterizing the cut;

Figure 3 is a schematic side view of a preferred embodiment of a diamond according to the invention;

Figure 4 illustrates the reflection of the lower part in the upper part in an embodiment of a diamond cut as a brilliant according to the invention;

Figure 5 shows the internal reflection of incident light on the girdle in an embodiment of a diamond according to the invention;

Figures 6A, 6B and 6C illustrate respectively schematic perspective views of an exemplary embodiment of a finished round diamond, of a finished round diamond with bezels which are twisted relative to the pavilions, and of a finished pear diamond according to the invention;

Figures 7A and B illustrate respectively the contours of a round diamond and pear diamond cut according to an embodiment of the invention compared to those of a conventionally cut round diamond and pear;

Figures 8A- 8D illustrate top views which are possible in an embodiment of the stone according to the invention;

Figures 9A- 9D illustrate possible top or bottom views of a pear diamond cut according to the invention;

Figures 1OA and B illustrate respective bottom views of an advantageous round diamond and of an advantageous pear diamond according to the invention.

A diamond is generally characterized by the presence of a table 1, a girdle 2 and a culet 3, as shown in Figure 2. The area between the table and the girdle is referred to as the crown 4. The area between the culet and the girdle is referred to as the pavilion 5. The crown and the pavilion are made up of facets 10-13.

The facets located between table facet 1 and girdle 2 are referred to as bezels. In the shown embodiment there are six bezels and each bezel comprises a table bezel facet 10 which adjoins table 1 and a girdle bezel facet 11 which adjoins girdle 2. The facets located between culet 3 and girdle 2 are referred to as pavilions. In the shown embodiment there are six pavilions: six culet facets 13 adjoining the culet and six girdle pavilion facets 12 adjoining the girdle.

In the embodiment of figure 2 two angles of inclination can be defined on the pavilion side: the girdle pavilion facet incline al, comparable to the pavilion incline as defined for a conventional stone, and culet facet incline a2, being the angle a2 which a culet facet forms with the girdle. Further present on the bezel side in this embodiment are two inclines: the girdle bezel facet incline βl, being the angle βl which a girdle bezel facet forms with the girdle, and table bezel facet incline β2, being the angle β2 which the table bezel facet forms with the girdle. Figure 3 illustrates the possible values for the average culet facet incline a2, which lies between 27 and 33 degrees, and for the average girdle bezel facet incline βl, which lies between 39 and 43 degrees. The variation from the average value normally amounts to no more than 10%, preferably no more than 5%, and most preferably no more than 1%.

Figure 3 illustrates a variant with table, although variants with a point as top end likewise lie within the scope of the invention. The table width 31 or size expressed as a percentage or fraction of the overall (average) width 36 of girdle 2 is preferably chosen between 10% and 30%, as illustrated in figure 3. The overall height 37 expressed as a percentage or fraction of the overall (average) width of the girdle is preferably chosen between 75% and 85%, as illustrated in figure 3.

Finally, the girdle thickness 32 is expressed as a percentage or fraction of the overall (average) width 36 of the girdle, preferably chosen between 2% and 10%, as illustrated in figure 3.

It will be apparent that the embodiments described here can be further finished (for instance by being cut as brilliants) without departing from the scope of the invention. Figures 6A, 6B and 6C illustrate three examples of a finished stone, here respectively a round diamond, a round diamond with rotation of the bezels relative to the pavilions, and a pear. Figures 6A and 6B show a round diamond with a table 102, a girdle 103 and a culet 104, and with:

- eight table bezel facets 126 and eight girdle bezel facets 125;

- eight girdle pavilion facets 114 and eight culet facets 115.

In the variant of figure 6B the pavilions are twisted through a twist angle y relative to the bezels. This angle can be further optimized in order to obtain the best possible reflection of the lower part in the upper part, taking into account the principles illustrated in figure 5, see below.

During the fine cutting work the star facets 123 and half facets 124 are arranged on the crown, and four additional facets 117, 118 were arranged in each case on the pavilion, although the skilled person will appreciate that there may also be more or fewer facets.

Figure 6C shows a variant in a pear shape, wherein corresponding facets are designated with the same numeral to which 1000 is added. This stone will again preferably be optimized in order to obtain the best possible reflection of the lower part in the upper part, taking into account the principles illustrated in figure 5, see below.

It will further be apparent to the skilled person that the girdle typically comprises a large number of small facets, which have been omitted in figures 6A, 6B and 6C in order not to overload the drawing.

Compared to the conventional proportion parameters (see Table 1) it is apparent that each of the described proportion parameters of the present invention differs therefrom. More specifically, in determined embodiments of the present invention the table is smaller and the overall height and the girdle thickness are usually greater than proposed for the conventional cuts. In possible embodiments of the present invention the culet facet angle a2 is likewise smaller than the conventional pavilion angle (typically 41 degrees) and the girdle bezel facet incline Sl is typically larger than the conventional bezel incline (typically 34 degrees) .

Despite these parameters differing from the conventional values, the described preferred embodiments are nevertheless characterized by improved optical characteristics which can be objectively determined by means of commercially available software applications.

The above described values for the angles of the culet facets and the girdle bezel facets, as well as the table size, will moreover allow at least the central part of the pattern of the pavilion to be reflected in the main facets of the crown, in the girdle bezel facets and in the table, as illustrated in figure 4. The central part of the pattern M of the lower part is reflected in the main facets 126', 126" of the upper part. This reflection is made possible by choosing a sufficiently small table in combination with suitable angles for the facets of the upper part and for the facets of the lower part. This pattern will typically further also be visible in table 102 ', 102" and in the lower facets 125', 125". This reflection is typically made possible by choosing a sufficiently small table in combination with a culet facet incline and a girdle bezel facet incline as stated in the preferred embodiments.

Depending on the flaws and the shape of the rough stone, the culet facet incline and a girdle bezel facet incline can be optimized for the best possible play of light providing for a sufficient brilliance, in other words having optimum optical characteristics. The described cuts are moreover characterized by a larger volume, with the result that the weight expressed in carats increases, which certainly provides an economic advantage. Figures 7A and B illustrate schematically the extra volume which can be obtained if a determined cut according to the invention is used. Cl indicates the contour of a traditional round diamond and C2 indicates the contour of a diamond cut according to the invention. The volume Oe gained is hatched. This volume will of course depend on the shape of the rough stone and on the flaws therein, but the skilled person will appreciate that in almost all cases a considerable gain in volume can be obtained with a cut according to the invention.

A possible cut of the present embodiment is designed such that the light incident in the diamond is reflected internally such that the reflection on the girdle is maximized. This principle as illustrated in figure 5 and makes a great contribution toward optimization of the optical characteristics. As a result of a suitable choice of the table width and of the angle a2 the light L exiting at the bezels will come largely from light reflected via the girdle and the culet facets. In order to obtain such reflections in the stone the angles d must be greater than 24 degrees for diamond materials. A suitable value of the angle a2 is particularly important in obtaining this effect.

An additional effect of determined cuts of the present invention is that the optical characteristics are optimized for light exiting not only on the table side of the diamond, but also on the culet side of the diamond. In other words, in respect of the evaluation of the optical characteristics there is not just one preferred orientation of the diamond but there are multiple preferable orientation options for the diamond. This allows the diamond with a cut according to the present invention to be set in different ways while retaining the optimum optical characteristics.

Finally, figures 8A- 8D show another three variants for the upper part of a stone according to the invention which are by no means limitative and which serve solely for the purpose of illustration. In the variants of figures 8A- 8C use is made of a relatively small table 102, 302, 402. For these variants the dimensions of the table will typically be between 1 and 40% of the overall diameter of the stone. In the variant of figure 8A use is made of star facets 123, and in each case two half facets 124 and a facet 125 which adjoin the girdle. In the variant of figure 8B use is made of small star facets 323 on each side edge of table 302. Provided in the variant of figure 8C are star facets 423 and facets 425 which adjoin the girdle. Figure 8D shows a variant in which the top end of the stone ends in a point 602.

Figures 9A-D further illustrate a number of possible and by no means limitative top or bottom views of a pear diamond according to the invention.

Finally, figures 1OA and B illustrate two further advantageous embodiments of the lower part of a round and pear diamond according to the invention. Corresponding lower parts can be used in any other type, any other shape or any other cut of diamond .

The lower part of figure 1OA comprises eight girdle pavilion facets 1014 which adjoin eight culet facets 1015. The stone is further finished with half facets 1010 which continue as far as culet 1004.

Figure 1OB illustrates a corresponding variant for a pear diamond. In this variant there are provided four girdle pavilion facets 1014' and four culet facets 1015 ' . The stone is further finished with half facets 1010' which extend between culet 1004' and girdle pavilion facets 1014'.

The invention is not limited to the above described exemplary embodiments, and the skilled person will understand that many modifications and variants are possible without departing from the scope of the invention, this scope being defined solely by the following claims. The present invention can be applied to any type, any shape or any cut of diamond, and thus to the existing, conventional fancy cuts (pear, cushion, princess etc.) as well as new, non-conventional and/or not yet existing fancy cuts.