The present invention relates to a cut diamond.

The quality and value of cut and polished diamonds for use as a jewel is often described by the four C ' s :

- Carat

- Clarity

- Colour

Cut

Carat is the unit of weight of diamonds, whereby one carat equals 200 mg. Traditionally, the value of a diamond is determined by its size or weight, expressed in carat. The bigger the stone, the higher the value .

Clarity refers to the imperfections and intrinsic impurities already present in the rough stone.

The colour is also determined by the rough stone. Colourless and transparent stones are scarcer and therefore more valuable . Cut refers to the cutting and polishing of the stone in a specific form with numerous facets. The manner a stone is cut determines the path the light, which enters the stone, will travel. Almost all the light that enters the stone will also leave it again. The reflections, refraction and dispersion along the light path will determine the brilliance and fire of the stone. Colour and clarity are intrinsic properties of the stone for which gradation scales have been defined. Their values cannot be changed to increase the value of the stone.

Traditionally, the stones are cut and polished in such a way as to retain their maximum weight during the method, while any defective spots, such as points or granules, could be removed. Because the play of the light in the stone was not co-ordinated, the light did not produce much fire or brilliance in the stone.

The last centuries the cut diamond's brilliance and fire have become more valued than merely the weight of the stone.

Even with modern techniques, cutting and polishing a diamond crystal always leads to a drastic loss of weight: said loss is rarely less than fifty percent.

If the crystal is an octahedron, the round brilliant cut is usually preferred because often two stones can be cut from one crystal then. As its name suggests, the brilliant cut is characterised by a lot of brilliance and fire.

In the mid-seventeenth century the first diamonds were cut in a form that was a precursor of the current brilliant cut, the so-called mazarins or double-cut brilliants. Since then the design of the brilliant cut has developed further.

The different dimensions and angles of a brilliant cut to maximise the brilliance were determined more by skill than by science. In 1912, Marcel Tolkowsky published his book "Diamond design, A study of Reflection and Refraction in Diamond" . This book contains a mathematical study to determine the optimal angles and dimensions for achieving the maximum brilliance with the brilliant cut.

Tolkowsky' s ideal model did not consider all aspects; it was a two-dimensional model and did not take into account the effects of the girdle. The model has been somewhat refined since. Nevertheless, study of diamonds that were supposed to be very well cut and polished, obtained by pure skill, showed that they have almost the same dimensions as predicted by Tolkowsky ¹ s ideal model. An example can be found in EP 1.181.875. An alternative construction for the traditional ideally cut brilliant can be found in US 20060086143 in which eight extra facets on the crown and sixteen extra facets on the pavilion are described.

We can conclude that the value of a stone with a given size can be influenced by changing the way it is cut . The goal is to obtain a maximum brilliance and fire in the stone while retaining the highest possible weight. The problem is that these are contradictory requirements.

It is therefore a purpose of the present invention to provide a solution which at least diminishes the aforementioned and other disadvantages .

To this end the present invention provides a cut diamond characterised in that it is a convex polyhedron and has at least sixty-four facets, which is one more than the fifty- seven facets of an ideally cut diamond brilliant when no culet is present.

In particular, the crown has twenty-nine facets, namely one heptagonal table facet, seven crown star facets, seven crown kite facets and fourteen half facets bordering the girdle.

The girdle can be provided with forty-two facets for example, but can also be executed cylindrically.

The pavilion consists of seven pavilion star facets, surrounded by fourteen pavilion kite facets, in turn delimited by fourteen pavilion girdle half facets, which border the girdle, or together thirty-five facets.

The seven lower star pavilion facets come together in the apex of the pavilion such that one asymmetric seven-pointed star shape is formed when viewed from below. There are sixty- four facets in total.

A cut diamond according to the invention strengthens the brilliance and the fire of the rough stone, with maximum retention of the weight.

A special aspect of the invention is the aspect that the cut diamond, viewed from above , generates a shift from refraction toward the blue, which gives a seemingly lighter colour and an extra brilliance that comes from the extra facets and the construction. The cut was called XIAO LONG, by analogy with small dragon, nickname of the last emperor of China or of Bruce Lee. With the intention of better showing the characteristics of the invention, a preferred embodiment of a cut diamond according to the invention is shown hereinafter by way of an example without any limiting nature, with reference to the accompanying drawings, wherein: figure 1 schematically shows a frontal view of an ideally cut brilliant;

figure 2 is a top view of the crown of the diamond of figure 1;

figure 3 is a bottom view of the pavilion of the diamond of figure 1;

figure 4 schematically shows a frontal view of a cut diamond according to the invention;

figure 5 shows a top view of the crown of the diamond of figure 4;

figure 6 shows a bottom view of the pavilion of the diamond of figure 4;

As the shape of a cut diamond according to the invention can most easily be explained based on the shape of an ideally cut diamond, we will first explain the geometry of such an ideally cut diamond.

An ideally cut brilliant 1 consists of fifty-seven facets when no culet is present. The shape of such diamond can be derived from the figures 1 to 3.

An ideally cut brilliant 1 consists of an upper part, called the crown 2, and a lower part, called the pavilion 3. These two parts are separated by a proportionally thin disk called the girdle 4. If no girdle is present, the horizontal plane that the crown 2 and the pavilion 3 have in common can still be called the girdle plane . The girdle 4 itself may or may not have facets, but this is not relevant for the present invention.

The crown 2 comprises thirty-three facets. The top facet of the crown 2 lies in a horizontal plane and is called the table 5. There are eight crown kite facets 6, eight crown star facets 7 and sixteen upper half facets 8.

The pavilion 3 comprises sixteen lower half facets 9 and eight lower main facets 10. The pavilion 3 may also contain a culet facet, which is a facet that is located near the apex of the pavilion 3 and is parallel to the table 5.

In an ideally cut brilliant 1, the axis through the middle of the table 5 and through the apex of the pavilion 3, or through the middle of the culet, if present, is an eightfold symmetrical axis. The shape of an ideally cut brilliant 1 can be described by eight independent parameters . Other ratios can be easily calculated from the independent parameters . The following table 1 summarises the most important parameters in the known way, whereby their planes and a reference are indicated in the drawings.

Table 1 Parameters of an ideally cut brilliant.

Less important is the size g, which indicates the minimum

5 height of the girdle 4, as is shown in figure 1. In the drawings an ideally cut brilliant 1 without culet is shown.

A diamond cut 11. according to the invention without a culet facet is also a convex polyhedron, but has at least sixty- four {= eight * eight, thus referring to the figure eight, which in Chinese culture stands for happiness) facets, of which twenty-nine on the crown side consisting of one heptagonai table facet, seven crown star facets, seven crown kite facets and fourteen half facets bordering the girdle .

There are thirty-five facets on the pavilion side, consisting of seven pavilion star facets, fourteen pavilion kite facets and fourteen pavilion girdle facets which border the girdle. Optionally, the girdle can still show forty-two girdle facets .

The axis running through the middle of the crown 2 and through the apex of the pavilion 3, is not an eight-fold symmetrical axis but runs through the middle of the table which forms an equilateral heptagon.

The following table II summarises the most important parameters of a preferred embodiment of a cut diamond 11 according to the invention, whereby their planes and a reference are indicated in the drawings.

Table II: parameters of the Xiao Long cut according to the invention.

5 Light that enters the crown cross in the diamond is dispersed and refracted. The refracted light falls on the thirty-five facets of the pavilion and is reflected. The pavilion 3 of a diamond cut according to the invention is usually deeper than that of an ideally cut brilliant 1, the effect being

10 that such a diamond holds the light in the diamond longer.

Viewed from above, the reflections form a bright diamond, which is characteristic for a diamond cut according to the present invention. At a cut angle of 36.0° for the crown kite facets (E) , the light reflection is optimal.

A diamond according to the invention may or may not have a faceted girdle 4. Both are seen as alternatives that fall under the protection of the present invention. In the case of a faceted girdle 4, the number of girdle facets is hereby determined in practice according to the state of the art.

Preferably, the axis running through the middle of the crown 2 and through the apex of the pavilion 3, or through the middle of the culet, if present, is not a symmetrical axis .

The present invention is by no means limited to the embodiments described as an example and shown in the figures, but a diamond cut according to the invention can be realised in all kinds of forms without departing from the scope of the invention, as is described in the following claims.