Patent Description:
The value of a cut diamond is determined to a significant extent by the four C's: cut, clarity, carat and colour. In addition to measuring weight, colour, size and cut, to evaluate the diamond, the inclusions must be located and their influence on the cost of the final brilliant must be minimized. The stone may have internal or external flaws or inclusions. Note that the terms "flaw", "inclusion" and "defect" are used as synonyms referring to a visually discernable irregularity inside the gemstone. There are various causes of inclusions, e.g. crystals of a foreign material, another crystal of the gemstone itself, or imperfections such as cracks which may appear whitish or cloudy. The clarity of a gemstone such as a diamond will depend on the number, size, colour, location, orientation and visibility of the inclusions. By locating and identifying inclusions within a rough (unpolished) gemstone, it is possible, in principle, to plan the cutting of the gemstone in such a manner as to minimize the impact of those inclusions upon the clarity of the final cut and polished stone.

The task of evaluating a diamond appears at almost all stages of the process from mining to selling. The choice of the evaluating method in a particular case depends on many factors. From the point of view of technique, historically the following methods may be distinguished:.

A cutter looks through the loupe to a diamond, trying to sort out the inclusions and their numerous reflections. He counts, identifies and localizes the inclusions approximately, weighs the stone, determines its shape, crystal structure, colour and clarity. Next he mentally cuts off the flaw zones and decides what he can get from the residual diamond using just his experience. This is the way to prize the rough diamond. Such a method is typically used when buying rough diamonds, when observing a stone to make a first draft of its cut, as a basic method for evaluating small size rough diamonds, or appraise the finished brilliants.

The cutter first observes the stone, and then opens a number (typically two, but sometimes one window is sufficient) "windows" on opposite sides of the stone so that by looking through one of them he can see the inclusion against the background of the other window. According to a first possibility he draws a line which is meant as a trace of the sawing plane. Finally, looking through one of the windows, he matches front and back lines and makes sure that the inclusion is left in the certain, predicted area of the diamond. According to another possibility two dots are marked on the opposite surfaces of diamond. Each couple of dots marks the beginning and the end of the line which goes through the inclusion. The intersection of the lines displays the position of the inclusion. Such a method is typically used at the stage of checking before sawing, to specify the location of the inclusion by looking from the different points of view, to check the appeared flaws in the middle of the process.

Typically, first a number of windows are polished in front of the inclusions. Note that in case of flat and smooth facets on the rough stone they may function as readily available windows. In a typical case, the windows must allow observing the inclusion(s) from at least two points of view (ideally orthogonal directions). Note however that for certain cases one window may be sufficient, see further. Next the stone is glued to a holder, mounted in a scanner and a 3D-model of the diamond is built. Further the process bifurcates depending on the used optics.

Working on the same holder, the inclusion is imaged from two (or more) sides as illustrated in <FIG>, see the viewing directions L1 and L2. Contours of the inclusion in the images represent the bases of so called "optical cylinders". Each cylinder extends between opposite locations on the outer surface of the stone in respective directions L1', L2'. The directions of the cylinders L1', L2' are calculated by the method of Snell's law, using the 3D-model of the diamond. The intersection body of the cylinders provides an approximate, usually accurate shape of the inclusion and its location. Such a method is disclosed in <CIT>. An example of a suitable device for performing this method is the "Helium Rough" device produced by the applicant.

As in the method under a), the inclusion in a stone <NUM> is imaged by an observer <NUM> through a microscope <NUM> from one, two or more sides, but the angles α1 and α2 (<FIG>) must typically be less than <NUM>° for an accurate result. In contrast to the method under a), each cylinder starts at the focusing point as illustrated in <FIG>. This allows to localize point inclusions from only one direction, and to determine accurately the inclusions of complicated form (even convex form) using just a few directions. Indeed, by using a microscope with a high magnification (low depth of field) the distance to a small inclusion can be derived relatively accurately from the distance between the lens and the focussing point, allowing the use of only one viewing direction for small inclusions. To increase the number of viewing directions additional freedoms for moving the stone are advantageous. This method is typically used for complex diamonds with a number of different types of inclusions. An example of a suitable device for performing this method is the "M-box" device produced by the applicant.

<FIG> illustrates the method used for a point inclusion, where a cylinder <NUM> begins at the focusing point and extends over a distance equal to the depth of field (DOF) in the viewing direction. The real shape of the inclusion is indicated with a dashed line, and it will be clear that the method of <FIG> will not give an accurate result for such an inclusion. <FIG> illustrates the method used for larger inclusions where a cylinder <NUM> begins at the focusing point and extends in the viewing direction to the outer surface of the diamond. This method will give good results for the illustrated inclusion. <FIG> show only one cylinder, but typically at least two cylinders are used, such that the shape and location of the inclusion can be determined by intersecting the at least two cylinders. Sometimes, for example in case of "cloud", i.e. a number of small inclusions close to each other, only one direction of observation is sufficient to localize the cloud by focusing to each of the point inclusions of the cloud step by step.

A diamond is glued to the holder and submerged in an immersion solution with the same refractive index as diamond. As a result, the diamond "disappears", but any inclusions remain visible. The stone in the immersion liquid is rotated and 2D images are obtained from a plurality of different viewing directions to determine a complete 3D-model of the inclusion(s). An example of such a method is disclosed in <CIT> and in <CIT>. <CIT> further mentions immersion of the stone in a solid material using infrared light and rotation scanning for determining the location of an inclusion.

In practice, often at the same technological operation different, both traditional and modern methods may be used. For important cases, a stone will typically be checked by eye (by method <NUM>) marking lines, wherein the stone has already been made automatically analyzed by any of the modern methods (<NUM> or <NUM>). To do such an eye check, it usually is necessary to open (polish) windows on opposite sides of the stone, and to look through both windows to make sure that the marking (sawing) line goes through the inclusion.

It is an object of the present invention to provide an improved method for analyzing a rough, semi-polished or polished diamond, in particular for determining the position of an inclusion in a diamond, for the purpose of e.g. evaluation, planning, cutting and/or virtual viewing.

According to an embodiment of the invention there is provided a method for analyzing a diamond according to claim <NUM> and a diamond taken up in a solid block of solid transparent material according to claim <NUM>. In the following, the term gemstone is used to illustrate the embodiments, while the claimed invention is limited to the analysis of a diamond.

By immersing the gemstone in a solid block it is not necessary to polish out windows in the gemstone itself, avoiding the risk that volume of the potential polished stone is decreased. This also means that after examination if the gemstone is under pre-purchase consideration and is rejected based on the inclusion evaluation, the gemstone can be removed from the transparent material and returned to its owner.

Further the method will allow to use all the traditional analyzing methods (see methods <NUM>-<NUM> described in the background section) in an improved way. Note that the solid block can be given any suitable shape (from the start or by additional polishing) in order to provide a good visibility of all areas of the diamond from one, two or more directions. Further, the use of a solid transparent material around the gemstone will facilitate the cutting process: the gemstone may be cut whilst remaining in the solid immersion material, together with the solid immersion material. Also, the planning marks may be put on the surface of the solid immersion material, avoiding marking lines on the surface of the gemstone. Further, visible range rays can be used for lighting the solid block with the gemstone inside. This allows the use of the same visible range optic devices as currently used in diamond industry for determining inclusions.

According to a preferred embodiment there is provided a method for locating an inclusion in a diamond, wherein the inclusion is visualized using light rays in the visible spectrum through a surface of the transparent block and wherein the position of the inclusion is determined using computer means. Typically a 3D model of the gemstone will be obtained before immersing the stone, and this 3D model will then be used in the calculations.

According to a preferred embodiment the gemstone is fixed on a holder, and the immersion material is applied whilst the gemstone remains fixed on the holder. This will allow to first scan the gemstone on the holder to obtain a 3D model of the outer surface in a pre-determined coordinate system, and next obtain a 3D model of the outer surface of the solid block in the same coordinate system. Also the holder with the solid block can then simply be transferred to the different devices used in the analyzing and/or cutting process.

According to another embodiment a number of morphological features of the gemstone are determined before the gemstone is taken up in the transparent solid block, and a number of reference points are chosen using the determined morphological details. After taking up of the gemstone in the solid block the same morphological features and associated reference points are looked up so that any analyzing done before including the stone in the immersion material can be matched to the analyzing done whilst the gemstone is contained in the solid block. To determine the morphological features a number of high magnification photographs may be used. Using such a method it is not necessary to fix the gemstone on a table or in a holder as in the previous embodiment.

According to a preferred embodiment the solid block is formed or polished with at least two flat surfaces. The solid block can for example be formed or polished with a prism shape, in particular a rectangular cuboid shape. Also, the solid block may be polished in order to obtain one or more improved facets for analyzing the gemstone. Further, the solid block may be polished such that a polished window is opposite a matt outer surface. This has the advantage that a homogeneous background is created for the view through the polished window. According to another advantageous embodiment the solid block is polished or shaped with a shape similar to the shape of the gemstone but with a number of flat surfaces. In such an embodiment the shape of the solid block can be such that the distance between the outer surface of the gemstone and the outer surface of the solid block lies within a predetermined range. Note that it is advantageous to reduce the distance between the gemstone and the outer surface of the solid block in order to reduce absorption and to obtain a better brightness when the gemstone is analyzed using light rays in the visible spectrum.

According to a preferred embodiment the solid transparent material has a refractive index RIs similar to the refractive index RIg of the gemstone, wherein preferably RIg - <NUM>,<NUM> < RIs < RIg + <NUM>,<NUM>, and most preferably RIg - <NUM>,<NUM> < RIs < RIg + <NUM>,<NUM>.

According to a preferred embodiment the solid material has a transparency for a predetermined segment of visible light, between <NUM> and <NUM>%, preferably between <NUM> and <NUM>% and more preferably between <NUM> and <NUM>%.

According to an embodiment of the method of the invention, before covering the gemstone with the transparent material, a 3D model of the outer surface of the gemstone is obtained relative to a coordinate system. After covering the gemstone with the transparent material, optionally the solid block is polished. Next a 3D model of the outer surface of the (optionally polished) solid block of transparent material is obtained relative to said coordinate system. According to a preferred embodiment, the analyzing comprises observing the inclusion through at least one facet of the solid block and obtaining at least one 2D image of said inclusion as seen through said at least one facet; and determining the location of the inclusion using said at least one image of the inclusion, the 3D model of the gemstone and the 3D model of the solid block. Preferably the analyzing comprises observing the inclusion through <NUM> or more facets.

According to a preferred embodiment the solid block is formed or polished with at least a first and a second surface through which an inclusion can be seen, wherein the analyzing comprises:.

Typically, before covering the gemstone with the transparent material, a 3D model of the outer surface of the gemstone is obtained relative to a coordinate system, and the location of the first and second surface of the solid block is determined relative to the same coordinate system. More in particular it will be advantageous to also build a 3D model of the solid block relative to the same coordinate system. The 3D model of the outer surface of the gemstone, the location of the first and second surface (or the 3D model) of the solid block, as well as the angle of incidence of the light rays and the refractive index of the solid immersion material can then also be used for determining the axial direction of first and second optical cylinder by applying the well known optical laws.

According to a possible embodiment a focusing function is used for obtaining the first and second 2D images of the inclusion, wherein the first 2D image is obtained whilst observing the inclusion in a first direction and focusing in a first focusing point and the second 2D image is obtained whilst observing the inclusion in a second direction and focusing in a second focusing point; wherein the first optical cylinder starts from an end surface containing the first focusing point and extends in the first axial direction, typically calculated using the location of the first surface; and wherein the second optical cylinder starts from an end surface containing the second focusing point and extends in a second axial direction, typically calculated using the location of the second surface.

As explained in the section above about the state of the art, if an optical instrument with narrow depth of field is used, it is possible to localize point inclusions from only one direction, and to determine accurately inclusions of complicated form (even convex form). For larger defects it is also possible to focus step by step on different portions of the defect to build a 3D model of the defect.

According to a preferred embodiment the gemstone is any one of the following: a rough diamond, a polished diamond, a semi-polished diamond.

According to a preferred embodiment the analyzing comprises marking the surface of the solid block providing indications for the cutting process and cutting the solid block using the markings on the surface.

According to a preferred embodiment a 2D image of the solid block is obtained, wherein the one or more inclusions are looked up in the 2D image of the solid block using an automated algorithm based on light/dark differences and/or based on the sharpness of the inclusions boundaries. This sharpness typically depends on focusing plane position: an inclusion will only have a sharp boundary when the inclusion is inside the depth of field.

According to a preferred embodiment the immersion material is an arsenic-sulphide composition, optionally alloyed with selenium or iodine, such as the arsenic sulphides (As(X)S(Y)), or arsenic sulphide iodides (As(X)S(Y)I(Z)), or arsenic sulphides containing iodine (As(X)S(Y)+(Z)%I), and for example As<NUM>S<NUM>, As<NUM>S<NUM>+<NUM>%I, As<NUM>S<NUM>, As<NUM>S<NUM>+<NUM>%I, As<NUM>S<NUM>I<NUM>, or a combination thereof.

The gemstone is diamond, and the immersion material is a material belonging to the class of glasses with RI = <NUM>,<NUM> +/- <NUM>,<NUM>, more preferably <NUM>,<NUM> +/- <NUM>,<NUM>, and most preferably RI = <NUM>,<NUM> +/- <NUM>,<NUM>.

According to a preferred embodiment the gemstone, whilst being analyzed, is heated up to a temperature below the melting temperature to decrease the inner tensions inside the solid immersion material. According to a possible embodiment thereof, the gemstone is analyzed with an optical device containing an infra-red heater for the heating of the solid block.

According to a preferred embodiment the solid transparent block is included in a container, in particular a cuvette, which is transparent in a segment of the visible spectrum or in a segment of the infrared spectrum. The analyzing is done through one or more walls of said container or cuvette whilst the solid transparent block is in the container. Including and analyzing the solid transparent block in a transparent container has the advantage that there can be chosen a suitable container with suitable facets and that no polishing of the transparent block is needed to obtain good facets. Further such a container can be easily cleaned before and during the analyzing which facilitates manipulation.

According to a preferred embodiment the melting or plasticizing of the solid immersion material is done in the container or cuvette, e.g. in an inert atmosphere such as Argon at a temperature of 195_-/+ <NUM> degrees Celsius depending on the selection of immersion glass material. Typically, the solid immersion material and the gemstone are placed in the container whereupon this assembly is heated to melt and immerse the gemstone in the immersion material and to obtain a good wetted connecting interface between the immersion material with the gemstone and the container. According to a preferred embodiment the assembly of the container with immersion material and gemstone is not cooled to room temperature but is kept at a higher temperature (but below the glass transition temperature) to avoid cracking of the glass. This may e.g. be achieved by providing the cuvette with an additional heating means, see further.

According to a preferred embodiment the container is manufactured of a quartz material. Such a material has the advantage that it has a relatively high heating coefficient.

According to preferred embodiment the container is a glass cuvette, e.g. a quartz cuvette, an IR quartz cuvette, an UV quartz cuvette.

According to a possible embodiment the container is heated during step of cooling (i.e. during the step of allowing the immersion material to become solid) and during the further analyzing to avoid that the container or the immersion material cracks during cooling. Note that this is only an option and that such a step will typically not be included if the immersion material does not crack during the solidifying step. According to a possible embodiment the container is provided with a heating means. However, according to other embodiments the heating may be performed by external means, such as circulating warm air, infrared heating, etc..

According to a possible embodiment the heating means may comprise an electrically conductive coating applied on the container or cuvette. This coating is typically at least transparent in a segment of the visible spectrum or in a segment of the infrared spectrum. Such a coating allows for a good heating of the immersion material. The coating is preferably manufactured from an Indium-Tin-Oxide material, but the skilled person will understand that other electrically conductive, optically transparent coatings may also be used. The coating preferably has a layer thickness which is less than <NUM> micron, preferably less than <NUM> micron, and most preferably less than <NUM> micron, and e.g. approximately <NUM> micron. In that way the refractive index of the coating material will not influence the optical paths in significant way and need not be taken into account. The coating may be applied on the inner surface and/or on the outer surface of the container. The coating may e.g. be applied by vacuum sputtering. Commercially available suitable coating materials include e.g. the ITO coating from Ocean Thin Films which is normally used for defrosting and maintaining LCD displays.

According to another variant the heating means may comprise one or more heating wires which may be arranged in or near the container.

By providing a heating means in the container it is possible to obtain an improved heating of the immersion material and a good interface between the container and the immersion material. Further the heating means may be used to heat the assembly of the container and transparent block during the analyzing.

According to a possible embodiment the gemstone fixed on a holder is included in the immersion material in the container. In such a case the holder may also be provided with a heating means. Alternatively the holder may be in contact with the coating so that it is also heated when heating the cuvette. In that way the holder can also be heated to avoid heat dissipation from the cuvette through the holder if the holder is inside the immersion material.

According to a preferred embodiment the container or cuvette has a polygonal shape with an open top, in particular a prism shape such as a beam shape, an octahedron shape or a pyramid shape. Such a polygonal shape has a plurality of edges, and one or more heating wires may be arranged at or near one or more of said plurality of edges. If the container or cuvette has a bottom, one or more heating wires may be provided at or near the bottom.

The invention also relates to a container or cuvette made of a material which is at least transparent in a segment of the visible spectrum or in a segment of the infrared spectrum, wherein said container or cuvette comprises a gemstone covered by a transparent solid material, which solid transparent material is adhered to the container or cuvette, in particular for use in an embodiment of the method of the invention. The container may have any one or more of the above disclosed features.

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

According to an exemplary embodiment of the method of the invention illustrated in <FIG>2F, the method comprises the following steps.

In a first step a gemstone <NUM> is glued on a holder <NUM>, see <FIG>. In a second step the gemstone <NUM> is measured (scanned) to construct a three dimensional (3D) model of the exterior surface of the gemstone <NUM>. Referring to <FIG> there is shown a gemstone imaging apparatus <NUM> comprising the holder <NUM> upon which the gemstone <NUM> is fixed; a light source <NUM>; and an imaging device <NUM> such as a digital camera. The holder <NUM> is typically capable of rotating and/or translating in order to reposition the gemstone <NUM> relative to the light source <NUM> and the imaging device <NUM>. The imaging apparatus <NUM> is communicating with a computer <NUM>. More in particular, the computer <NUM> can control the holder <NUM> and the imaging device <NUM> in order to receive images of the gemstone <NUM> in different positions thereof. While not shown in <FIG>, the imaging or scanning apparatus <NUM> may include additional components for performing additional measurements of the gemstone <NUM>. Various scanning apparatuses generally suitable for use with the present invention are known in the prior art.

In a third step illustrated in <FIG>, the stone <NUM> on the holder <NUM> is immersed in a liquid material suitable for forming a transparent solid encasing around the stone. The liquid glassy material is then cooled to form a transparent solid body around the stone. The information about the location of the diamond inside the solid material can e.g. be kept either by the holder or by special marks put on the sides of the solid immersion beforehand. After solidification of the preformed molten medium and, optionally, after further polishing, the medium can have the form of a cube, an octahedron, or other forms similar (or approximately equidistant) to that of the diamond with well-polished sides. An example of an octahedron shape similar to that of the original diamond is illustrated in <FIG>. The solid immersion material SI is shaped with an outer surface OS in the form of an octahedron. A small distance dmin between the solid immersion polished side plane and the surface of the diamond provides a better contrast and brightness of the inclusion thanks to minimizing the light scattering in the small layer of the immersion.

Note that the 3D model of the outer surface of the stone has been determined with respect to the holder and that the shape of the encasing with respect to the holder is known or can be determined, such that a 3D model of the outer surface of solid encasing can be built on the same coordinate basis as the coordinate basis used for the 3D model of the gemstone. According to a possible embodiment illustrated in <FIG> a scan of the outer surface of the solid block <NUM> is performed in order to build a 3D model of the solid block on the same coordinate basis as the coordinate basis used for the 3D model of the gemstone. Using these two 3D models and the refractive indexes of the material of the gemstone and of the solid encasing the light paths within the solid block can be calculated.

Preferably, the solid block <NUM> applied around the diamond by immersion has a number of flat surfaces through which the diamond can be observed, and can e.g. have the shape of a prism, such as a rectangular cube. According to another possibility the solid block may have a shape which is similar to the shape of the gemstone inside but has a number of flat surfaces, see e.g. <FIG>. More in particular the shape of the solid block could be such that the distance between the outer surface of the gemstone and the outer surface of the solid block lies within a certain range. Note that it is possible to change the shape of the solid block e.g. by polishing, and that is e.g. possible to add additional facets to the solid block for an improved viewing of the inclusions, and this without removing valuable gemstone material. This is illustrated in <FIG>. In the illustrated embodiment two windows <NUM> are polished on the solid block <NUM>. In such an embodiment the steps illustrated in <FIG> and <FIG> will of course be performed on the solid block <NUM> after polishing the windows <NUM>. Further, it may be advantageous to combine a matt surface <NUM> with one or more views through polished surfaces <NUM>. Polished windows may e.g. be used to find and build inclusions, wherein a matt surface has the advantage of creating a homogeneous background for the view through the polished windows <NUM>. In other words, in certain cases, it may be advantageous to make sure that there is a matt surface <NUM> opposite a polished window <NUM>.

Preferably the casing <NUM> should be transparent in a part of the visible spectrum, with a transparency of between <NUM> and <NUM>%, preferably between <NUM> and <NUM> %. This will allow the use of the currently available cameras. Such a suitable material will typically be a glassy material with a refractive index similar to the refractive index of the gemstone in the solid state thereof. For diamond, the RI of the glassy material is <NUM> +/-<NUM>,<NUM>, and more preferably <NUM>,<NUM> +/- <NUM>, and most preferably <NUM>,<NUM> +/- <NUM>,<NUM>.

Examples of suitable materials are arsenic-sulphide compositions, preferably alloyed with Selenium or Iodine, e.g. arsenic sulphides (As(X)S(Y)), arsenic sulphide iodides (As(X)S(Y)I(Z)) or arsenic sulphides containing iodine (As(X)S(Y)+(Z)%I). More in particular, examples are As<NUM>S<NUM>, As<NUM>S<NUM>+<NUM>%I, As<NUM>S<NUM>, As<NUM>S<NUM>+<NUM>%I, As<NUM>S<NUM>I<NUM>, or a combination thereof. The skilled person will understand that the values of X, Y and Z in the formulas given above can be optimized to obtain a suitable refractive index in the solid state and suitable thermal expansion and plasticity properties. Preferably, the material has a low melting point (e.g. about <NUM>-<NUM>). Further, preferably, the material has a coefficient of the thermal expansion (CTE) close to that of the gemstone and/or a plasticity which is sufficiently high to avoid cracking after solidification at room temperature.

For diamond, the immersion material is a material belonging to the class of glasses with RI = <NUM>,<NUM> +\- <NUM>,<NUM>, more preferably, <NUM>,<NUM> +/- <NUM>,<NUM>, and most preferably <NUM>,<NUM> +/- <NUM>,<NUM>. In particular, the arsenic-sulphides alloyed by selenium or iodine may be used, optionally heated up to the temperature close to the point at which the material is about to soften, but still solid. Heating the material slightly whilst keeping a solid immersion body will decrease drastically the inner tensions inside the solid immersion material. This will allow avoiding knags and cracks, which could appear because of the difference in thermal expansion coefficient between diamond and the solid immersion material. The solid immersion materials mentioned above are not toxic at such temperatures (typically about <NUM>), and the temperature still allows the use of the currently available optical devices which may be slightly modified by adding a small standard infra-red heater for obtaining those increased temperatures.

In a further step illustrated in <FIG>, the glass block <NUM> in which the gemstone <NUM> is taken up, is placed in an imaging apparatus <NUM> for determining the location of the inclusion in the gemstone. More preferably, a scanning apparatus of the general type disclosed in <CIT> may be utilized in embodiments of the present invention. Possible methods use optical cylinders and are similar to the methods of the prior art illustrated in <FIG>. If the RI of the material of the solid block is identical the RI of the gemstone, it is possible to simply use the methods illustrated in <FIG> with this difference that the 3D model to be used is the 3D model of the solid block and not the 3D model of the diamond.

If the RI of the material of the solid block is different to the RI of the gemstone, the method is slightly more complex as illustrated in <FIG>. More in particular, both the outer surface A of the solid block SB and the outer surface B of the gemstone D need to taken into account to calculate the trace of the beam between an inclusion I and an observer O. The coordinates of the surface are known from the 3D-models which are determined as explained above. Further, <FIG> shows in detail how to build the shape of the inclusion by intersecting optical cylinders C1, C2. In the case of a narrow depth-of-field apparatus (cf. <FIG>) the cylinders extend in the direction of the light trace between the focal plane and the outer surface of the gemstone, and have a shape closely fitting the contour of the inclusion, see the dark hatched areas. If "direction" is defined as the perpendicular to the side through which the inclusion is observed then, when using a narrow depth-of-field apparatus, each direction typically allows measuring through it within a cone with a vertex angle of <NUM> to <NUM> degrees. Increasing this angle will result in a decrease of the measurement accuracy due to the astigmatism phenomenon. When a wide depth-of-field apparatus is used (cf. <FIG>), cylinders C1, C2 are created extending in the direction of the light trace from one location of the outer surface to an opposite location on the outer surface and having a shape closely fitting the contour of the inclusion, see the light and dark hatched areas in <FIG>. Further, according to a preferred embodiment the phenomenon of astigmatism (the deformation of the width of the ray bundle after refraction) can be taken into account in the calculation, as illustrated in <FIG>.

In <FIG> it is shown how the RI of the solid glassy material influences the image of the stone. In the example the gemstone is a diamond (RI = <NUM>,<NUM>) and the RI of the solid glassy material is <NUM>,<NUM> (<FIG>) and <NUM>,<NUM> (<FIG>). It is immediately apparent that a value closer to the RI of the gemstone will facilitate the localizing of the inclusion.

<FIG> shows how the glassy material helps to detect an inclusion (encircled) which are barely noticeable without any immersion (<FIG>). More in particular, the inclusion is more clearly visible in the photo of <FIG> then in the photo of <FIG>. This shows that the method of the invention is also appropriate to use in the event that RI is not exactly equal to that of the gemstone.

Embodiments of the method of the invention have amongst others, one or more of following advantages over the prior art:.

<FIG> illustrates an embodiment where the solid immersion material <NUM> is included in a cuvette <NUM>. This cuvette <NUM> is transparent in a segment of the visible spectrum or in a segment of the infrared spectrum, and is e.g. manufactured of quartz. The analyzing of the gemstone can be done through one or more facets of said cuvette <NUM> whilst the solid transparent block is in the cuvette. To include the gemstone in the immersion material in the cuvette, a piece of immersion glass is put into the cuvette. Then the assembly of cuvette, immersion material and gemstone is heated in an inert atmosphere such as Argon at a temperature up to <NUM> -/+<NUM> degrees Celsius depending on the type of immersion glass material selected. It is preferred if the assembly is not cooled down to room temperature but kept at a slightly higher temperature for the analysis of the gemstone in order to avoid cracks in the immersion or cuvette glass. To achieve this, the cuvette may be slightly heated, e.g. up to <NUM>-<NUM> degrees Celsius but below the immersion glass solid/liquid transition temperature. In that way the immersion material becomes more plastic and problems with differences of the temperature expansion coefficient between the cuvette glass and the immersion glass are avoided. Also there a good wetted contact interface is achieved between the immersion material and the cuvette.

<FIG> illustrates two possible shapes for a cuvette 140a, 140b. The cuvette 140a has a beam shape, and the cuvette 140b has an octahedron shape with an open top. Such a polygonal shape has a plurality of edges, and one or more heating wires <NUM> may be arranged at or near one or more of said plurality of edges. According to another variant an electrically conductive coating may be provided on the inner and/or outer surface of the cuvette, as disclosed above in the summary of the invention. Further there may be provided electrical contacts to connect the coating to an electrical source such as a current source.

Claim 1:
Method for analyzing a diamond, typically for locating an inclusion in a diamond, comprising:
- selecting a solid material which is transparent in at least a segment of the visible spectrum or a segment of the infrared spectrum;
- melting or plasticizing said solid material and immersing the diamond in said melted or plasticized material such that the diamond is at least partly covered therein; wherein the solid transparent material has a refractive index RIs and the diamond has a refractive index Rig, wherein RIg - <NUM>,<NUM> < RIs < RIg + <NUM>,<NUM>;
- allowing said melted or plasticized material to become solid such that a solid transparent block is obtained containing the diamond; characterized in that the method further comprises:
- analyzing the diamond through a surface of the transparent solid block using light rays in the visible spectrum or in the infrared spectrum, wherein the analyzing comprises submerging the solid block in an immersion liquid.