WO1998008081A2

WO1998008081A2 - Method of identifying cut diamonds

Info

Publication number: WO1998008081A2
Application number: PCT/EP1997/004112
Authority: WO
Inventors: Roland Diehl; Nikolaus Herres
Original assignee: Roland Diehl; Nikolaus Herres
Priority date: 1996-08-02
Filing date: 1997-07-29
Publication date: 1998-02-26
Also published as: DE19631367C1; WO1998008081A3

Abstract

The invention concerns a method used for radiographically identifying diamonds, in particular for determining whether a completely cut diamond is identical to or different from a known completely cut diamond. To that end, the position of the table facet of the diamond relative to the diamond lattice is determined by means of two crystallographic polar angles and then at least one radiographical transmission recording of the internal defects of the diamond is realized by monochromatic X-radiation at the Bragg angle. The identity of the diamond can be checked by comparing the polar angles determined and the X-ray topograms with polar angles and X-ray topograms of known decorative diamonds obtained by the same procedure.

Description

Process for identifying cut diamonds

The invention relates to a method for the X-ray identification of cut diamonds, in particular for determining whether a fully cut diamond is identical to or different from a known fully cut diamond.

Diamonds cut for jewelry purposes (brilliants, navette, drops, triangles, trapezoids, baguettes, emerald cuts, etc.) generally represent a high monetary value. Their value is determined by weight, color, purity and cut quality. These four value characteristics of a cut diamond are determined by a professional diamond grading to be carried out according to internationally established rules. In the case of cut jewelery diamonds of higher value, the weight of which is usually above 0.5 carat, the result of the graduation is recorded in the form of a certificate in a traceable form, which is enclosed with the graduated diamond. Due to the information contained in the certificate, each cut diamond is given an individuality which, in the event of loss due to carelessness, theft, etc., usually allows it to be recognized if it is found again. This recognition is questioned by the slightest changes in the cut and thus also in the weight of the stone,

REPLACEMENT BUTT (REGR26) yes made possible if there are no other typical features, such as the nature of a characteristic inclusion

In the past, efforts to develop a characterization process that made a cut jewelry diamond distinctive were lacking in the past. The nature of its crystal lattice, which is characterized by individual structural defects, remains unaffected by external manipulations on the stone. These internal structural defects can be caused by diffraction of X-rays on the crystal lattice of the diamond be made visible and recorded on X-ray-sensitive films or with other recording techniques.This kind of image of Kris's overall defects (X-ray topogram) is to a certain extent the fingerprint of the examined diamond and as such distinctive, since - similar to the fingerprint in humans - there are practically no two diamonds that match the X-ray topogram supply A X-ray topogram would be preserved on microfilm, attached to the certificate and a copy of it in an archive. If the stone is lost or is it lost? stolen, so once recovered, a new X-ray topogram can be used to determine its identity beyond any doubt

The unique and unmistakable identification of raw, partially or completely cut jewelry diamonds by means of X-ray topography (finger pinting) has been known for many years (see e.g. A. R Lang et al., Fingerprmtmg Diamonds by X-ray Topography, Industrial Diamond Review, Maren 1976, S 96-103)

In the practice of trading in high-quality cut diamond diamonds, it has so far not prevailed, since the procedure used according to the state of the art for the creation of the pontoon topograms is too cumbersome and too complicated

REPLACEMENT TT (RULE 26) Many X-ray topograms of a diamond are recorded so that with a single control topogram (daughter topogram) the proof of identity is beyond doubt. In US Pat. No. 4,125,770 "Diamond Identification" dated November 14, 1978 and German patent DE 25 59 245 C2 "Method for identifying diamonds" dated May 26, 1988, which state the prior art of X-ray topograms for identification to describe cut diamonds in their "lineage" from rough diamonds, the use of monochromatic X-rays in the "simplest routine procedure" (simplest routme) assumes a number of twelve increasing projection topograms, the diffraction of the X-rays at network planes of the type {400 } is made because cube faces have the least multiplicity. Using the symmetry, four X-ray topograms are produced one after the other after adjustment of such a network plane by diffraction at {400} network planes perpendicular thereto, the crystal being rotated 90 ° around the network plane normal between the exposures. The exact direction of pro ection remains open in the two patents.

The direction of projection is given by the path of the X-ray beam diffracted at the crystal lattice in the diamond. It is determined by the beam reflecting network (hκl) in connection with the azimuthal orientation of the diamond crystal around the associated network plane normal [hkl]. This means that the position of the irradiated crystal in the "dispersion plane" (= plane of the incident and diffracted X-ray) only by specifying the network plane array used (hl) and in addition a crystal 11 l chung [uvw ], which eg is perpendicular to the dispersion plane, is defined. However, the method only works reliably, i.e.

ERSATZBUVTT (RULE 26) The mother and daughter diagrams will only be practically identical if this direction of production is also specified, for example by setting a <110> or <100> direction perpendicular to the dispersion level. In any case, only one daughter topogram is used for unambiguous identification A total of at least 12 mother topograms are absolutely necessary, because 3 network levels of type {400} are to be used to produce topograms in 4 designations.

Our own experiments have shown that even this number is not sufficient for unambiguous identification by 1.1 comparison of the mother and daughter topogram. According to Friedel, the diffraction conditions for direction and opposite direction remain the same in a centrosymetric crystal lattice, but the projection conditions change, since the volume of the diamond is irradiated differently in the direction and opposite direction. Such X-ray topograms can therefore differ from one another due to the different geometric superimposition of the construction error contrasts and as a result of the absorption in the case of larger crystals. To get a complete fingerprint of the stone you had to take four pictures around each of the three cube axes in a "positive" direction and e four pictures around each of the three cube axes in a "negative" direction. This means the recording of a total of 24 x-ray topographs ensured that a single daughter admission was sufficient to achieve complete agreement with one of the 24 mother admissions.

The entire procedure requires qualified personnel, a lot of work and time and generates prohibitively high costs. This is the reason why the fingerpriπting of valuable cut jewelry diamonds, although highly desirable to protect property, has so far not been able to establish itself as a commercial sub-technique of diamond grading

REPLACEMENT BUTT (RULE 26) The object of the present invention is to create a method with which it is possible to identify cut diamonds in a comparatively simple and quick manner.

To solve the problem it is proposed that with the aid of a goniometer with five circles (29, Ω, Λ, Σ, .-) with monochromatic X-rays in an X-ray diffraction adjustment routine, the position of the table facet of the diamond relative to the diamond lattice is determined by two crystallographic polar waves (ρ, cp) is determined

an orientation of the diamond is determined from this,

subsequently, with this orientation, at least one X-ray topographical transmission image of the internal defects of the diamond is made by means of monochromatic X-ray radiation at Bragg's angle,

and the identity is checked by comparing the determined pole angles and X-ray topograms with pole angles and X-ray topograms obtained using the same procedure and known cut diamonds.

This provides a much simpler method for the X-ray diffraction / X-ray topographical characterization of cut jewelry diamonds. Here, X-ray diffractometry determines two angles ("pole angles") on the test specimen with high accuracy and then, as a rule, creates a single mother topogram. Together with the two determined angle information, this mother topogram is enclosed with the diamond graduation certificate on microfilm or another data carrier in order to prove identity or non-identity with a single daughter topogram recorded according to an identical procedure. More than a mother topogram will only be necessary for a very few cases that can be precisely specified, although it is advisable to make a maximum of four mother topograms, for example, for diamonds with a low defect level.

SPARE BLADE (RULE 26) A goniometer with five circles (2Θ, Q, Λ, Σ, Δ), an X-ray source and a sample holder as well as a detector are used as the adjustment device.With the help of a preferably motorized goniometer, with the participation of some or all circles, a special control program enables fully automatic orientation of the test object relative to the X-ray beam, the angle measurement can be carried out precisely on the circles ^~ and Δ and the topogram / topograms for shadow-free defect imaging in transmission can be made up to an asymmetry angle of 40 °.

The procedure can be performed routinely by technical personnel. Compared to the prior art, extensive automation and considerably reduced time and personnel expenditure for the new method open up the economical application of the X-ray topographical identifier (F gerprmting) in the practice of certification of high-quality cut diamonds. It is also suitable for identifying synthetic ones due to specific growth characteristics Diamonds, which in the future will increasingly appear in interesting sizes, colors and purities in the diamond trade. Embodiments of the invention are listed in the subclaims.

The invention is described in more detail below with its essential details.

It shows :

1 shows a stereographic projection of the crystal directions <100>, <110> and <111> of a diamond with marked areas "A" and "B" in the spherical standard triangle, 2 shows a goniometer with the position of the goniometer axes (2θ, Ω, X, Σ, Δ) and orientation of the diamond crystal at the start of the search for the reference direction,

3 the position of the normals on the table facet, given by the pole angles p and ψ in the standard triangle of the stenographic projection,

4 shows a stereographic projection of the position of symmetrically equivalent reflections of the type hkO on the zone circle of the reference direction <100> in the event that the more precise pole angle ρ _{mes 100} is greater than a limit angle _value ρ _{lιm resulting} from the measurement accuracy of the goniometer, where the reference direction is in the center of the projection and the pole of the table facet is deflected by the polar distance p ₁₀₀ with respect to the reference direction with the azimuth φ _10C ; the angular distances c between the pole of the table facet and symmetrically equivalent (hkO) poles are clearly distinguishable;

5 shows a stereographic projection of the position of symmetrically equivalent reflections of the type hkO on the zone circle of the reference direction <110> for the case that p _{mes no} > g _h -, the reference direction being in the center of the projection and the pole opposite the table facet the reference _direction with the azimuth p _{u0 is deflected} by the _pole distance p _uo ; the angular distances ε_ between the pole of the table facet and symmetrically equivalent (hkO) poles are clearly distinguishable;

6 shows a stereographic projection of the position of symmetrically equivalent reflections of the hkO type on the zone circle of the reference direction <100> for the case that p _mβ . ₁₀₀ < W

ρ, _ _m , where the reference direction is exactly in the center of the projection and the pole of the table facet is practically indistinguishable at the same location and

7 shows a stereographic projection of the position of symmetrically equivalent reflections of the type hkO on the zone circle of the reference direction <110> for the case that ρ _{mes 110} <ρlim, the reference direction being exactly in the center of the projection and the pole of the table facet practically not distinguishable in the same place.

As the practice of diamond grinding shows, the rough stone is usually delivered as a diamond octahedron, which is chopped into a large and a small rough diamond parallel to the surface of the cube {100}. The saw surface becomes the later table facet for both raw parts. Even if the rawest is a rhombic dodecahedron, it is usually said along the surface of the cube. So it happens that in about 80% of all polished jewelry diamonds, the axis of the cube and the normal to the table facet are approximately parallel and at most include an angle c _λ <10 ° with irregularly shaped rough stones or those whose cut is different due to the material economy larger angles can result between the cube axis and the table normal. In principle, the plate normal always falls in a solid angle, which is limited by the <100>, <111> and <110> delimitations. This is shown in FIG. 1 with the aid of stereographic projection ("Wulf f ^* net"). Because of the high symmetry of the diamond, all of the following considerations can basically be traced back to this spatial range (spherical "standard triangle").

In the vast majority of cases, the plate normal in the vicinity of a <100> direction (cube axis) will cut out. In the Projection topography is the distortion level of the cut diamond as little distorted on the X-ray topogram to maximize the projected cross section and thus the information content of the topogram. That <-100> direction that forms the smallest angle with the table normal therefore becomes the crystallographic reference direction for the determination of the pole angle and the unambiguous adjustment of the crystal before the recording of topograms

In such cases, if the angle between the <100> direction and the table normal is greater than 27 °, the table normal no longer falls within the range A "in FIG. 1, with the result that the girdle plane has been very strongly distorted on a transmission X-ray topogram In this case, among the neighboring <110> directions, the one with the smallest angle to the table normal to the crystallographic reference direction is explained. For the subsequent topographic image (s), only those reflections are used that lie on the zone circle for the respectively selected reference direction

The process flow can be divided into six steps

1 The angle between the normal of the table and the subsequent <100> or <110> direction is determined diffractometrically in order to determine the crystallographic reference direction and the further procedure. For this purpose, the diamond 4 is mounted in the center of a special 5-circle goniometer 1 (with the axes 29, Ω, X, 7, and Δ) in the zero position (FIG. 2). In this position, the table facet of the diamond is parallel to the rotating circles X. and Δ, Ω-axis and Σ-axis are parallel to each other and lie in the plane of the X and Δ rotation circles.

Here, as in the following, the axes of the goniometer 1 10

with large Greek letters (2Θ, Ω, X, 1, and Δ), while pole distance p and azimuth φ of the table facet as well as the rotary movements around the goniometer axes (2β, _' ,.,' - and o) with small Greek letters be designated.

The reference is found using a search procedure with systematic rotary movements of some or all of the goniometer circles, in that a suitable reflex (e.g. 400th) with a monochromatic X-ray radiation from an X-ray source 2 in symmetrical Theta / 2Theta (θ / 2 θ) reflection or 220) is searched for and precisely adjusted.

The search procedure as an adjustment and measurement routine can be carried out manually or more fully automatically using computer-controlled special software. The signal from the X-ray detector 3 is recorded, evaluated by the computer and used to optimize the angular positions.

In the end position there is either * 100> or <-110> as the crystallographic reference direction in parallel to the X axis of the goniometer. Now the angle n can be read on the goniometer. It corresponds to the deflection of the normal to the table facet from the reference direction and thus indicates the pole distance ρ ₁₀₀ (or u, _) of the table facet (FIG. 3).

Now the azimuth φ of the table facet normals is determined. Starting from a basic position (Y = 0 °), the goniometer and detector arm are set to symmetrical '~' / 2θ transmissions geometries for reflections of the hkO type, the poles of which lie on the zone circle with respect to the reference direction (eg 220). Knowing the 220- The reflex position on the zone circle to the reference direction now allows the exact positioning of the table normals in the standard triangle of the stereographic projection. The position of the table normals can be specified in the event that <100> was used as the reference direction by the pole angle, and Φ _tf -, or if the reference direction <110> was used, taken together by Q _^ and _PO , the jealous angles p and definieren clearly define the position of the table normals of the test specimen in relation to the crystallographic reference direction (FIG. 3).

The two pole angles can be reproducibly set at any time within the goniometer tolerances.They are now used to determine the orientation of the decorative diamond for the X-ray topographic image and the number of required X-ray topograms and thus indirectly the accuracy of the pole angle reading As the critical angle (mainly due to manufacturing technology) is currently the pole distance. "= 0.2 ° realistic, the _critical angle for the azimuth cf _lxm is also dependent on the deflection o For o = 0.3 ° Φ, _{τ is} currently around 3 °, for o = 10 ° around 0.2 °; below ρ _m a <p _lιm no longer makes sense φ _llιr is determined by the control software as a function of o _lιm . The number of minimum mother topograms to be recorded is determined by the deviation of the measured pole angles ς _mes and φ _mes from their critical angles.If the symmetrically equivalent reflections on the zone circle of the reference direction can be distinguished by the angle ε, which the relevant network planes include with the table facet (Fig. 4 and 5), is usually (probability> 95%) by the already at low apparatus-related effort, quite precise determination of the pole angle, only a single X-ray topogram may be required. As a criterion for the choice of the reflex used for this one X-ray topographical image, the angle ε between its network plane normal and the table facet normal is practically used. The automatic process sequence can, for example, be set up in such a way that among the symmetrically equivalent 220 network planes on the zone circle of the reference direction, the network plane is selected for the imaging whose angle deviates the least by 90 ° with the table facet (compare angle f. In FIGS. 4 and 5).

If the angles ε _{1 #} which include symmetrically equivalent reflections on the zone circle of the reference direction with the table facet are indistinguishable, so in rare cases the table normal coincides with the reference direction within the goniometer accuracy (Fig. 6, 7) more than one mother topogram is required, namely a maximum of four for reference direction <100> and a maximum of two for reference direction <110>. Now the method proposed here has already saved two thirds of the adjustment, measurement and evaluation time compared to the prior art and additionally provided the information that can be used for the certification that the angle ρ _mes between the standard facet and the reference direction is less than ρ _m .

4. Now the required X-ray topograms are created. An X-ray topogram can be recorded conventionally using the tried-and-tested long topography, ie using point focus and oscillating translational movement of the irradiated sample and the recording medium (X-ray film, nuclear trace emulsion, Multi-channel plate, CCD camera, etc.) relative to the incident X-ray beam. Alternatively, with the continuous improvement of the spatial resolution of CCD (charged-coupled device) image recording systems, a modified recording method will be preferable due to the much simpler handling (including elimination of the wet chemical process steps), higher speed and easier archiving options. In this, the X-ray beam incident on the sample is e.g. B. expanded by Bragg reflection on now commercially available X-ray mirrors (so-called. Goebel mirror or Gutman Optics) in two dimensions and parallelized and shines through the sample in full width and height at the same time, ie the translation of sample and recording medium relative to the X-ray beam can remain under. The electronic image recording system connected downstream of the CCD is able to compensate for any locally inhomogeneous change in intensity in the X-ray beam incident on the sample by forming a difference or in some other suitable way. However, the following explanations relate to the most high-resolution method in the current state of the art: recording medium = nuclear trace emulsion. To create the X-ray topograms, the preselected reflex, for example of the 220 type, can be approached by computer and optimized with regard to the angular positions via the detector signal. The adjustment panels are then removed and anti-scatter panels are set up. Then, with the X-ray switched off, the film cassette is placed and the (preset) oscillating translation movement is switched on. The width of the oscillation corresponds to the greatest width of the diamond parallel to the direction of translation. The X-ray beam is switched on, shines through the crystal and exposes the film in the light of, for example, a 220 reflex for a defined time (Timer)

If X-ray topograms of additional 220 reflexes are required according to the criteria given above, the film is temporarily removed after the first exposure. Another 220 reflex is adjusted and the width and position of the lens hoods are adjusted if necessary. The subsequent exposure of the film is carried out using the same method as the previous one. Conveniently, the still undeveloped film from the first shot is reused by making sure that the new shot does not overlap with the previous one. For this purpose, the film cassette can be moved by a few millimeters behind an aperture. After the necessary exposures on the X-ray film have been carried out under approximately the same exposure conditions, the film piece as a whole is developed and fixed at once for reasons of simplicity, especially in later identification (pattern recognition ), the method should preferably only be operated with one reflex type (i.e. only 220 or only 400).

5. The determined angle information (reference direction, pole angle, critical angle) and X-ray topograms are attached to the certificate of the diamond testing agency as microfilm or in some other way and a copy is kept in the archive. Ideally, the creation of a central archive is recommended.

6. If a polished diamond that has been lost due to pole angles and mother topogram (s) is recovered, the pole angles are measured and a daughter topogram (control recording) is recorded using the same adjustment procedure. In the case of identity, the daughter topogram agrees with that 15

Mother topogram or one of the mother topograms. The pole angles now have two functions: on the one hand, they are parameters of the test specimen, which become worthless only by changing the angle of inclination of the table relative to the crystal lattice, and on the other hand, they serve as search parameters for quickly finding the (mother) topograms, which can be found about these two Have numerically understandable sizes conveniently located in the archive. For this purpose, a subset ρ _fll and φ _{fil are} screened out as search parameters of the archive, for which the following applies:

yiii. ≤ Otii ≤ P _mes + &:>.

mes - Ψlim ≤ Φfil ≤ Φmes + <plιm

The search can also be carried out automatically using suitable software.

With an increasing number of mother topograms, pattern recognition using electronic image processing within the subset with similar pole angles p and <ρ can be useful. The check is carried out in essential details with the mother topogram of the (central) archive, i. H . of contrasts due to internal crystal defects. A complete agreement of all picture details, i.e. also the contrasts due to polishing damage (e.g. depiction of damaged facet edges) will of course only exist if no mechanical changes have been made to the jewelery set in the meantime.

If the orientation of the table facet was changed relative to the crystal lattice of the diamond (only possible by grinding the table), the crystal can still be identified in principle using the method described, although this may be complex. If the board is slightly sanded (<2 °), the search can be carried out in the archive 16

(Pattern comparison) to be extended to neighboring pole winnings. Stronger regrinding (e.g. after parts of the jewelery diamond) makes identification difficult in such a way that up to 24 X-ray topograms had to be made retrospectively. However, the effort involved could only be accepted in exceptional cases. Naturally, a single X-ray topogram can only guarantee reliable identification if the jewelry diamond is "within the scope" of its defect content. X-ray topograms of very defect-free crystals may offer Except for growth streaks with low contrast, only a few usable characteristics. In contrast, defective crystals can useless contrasts by superimposed projection be worthless. In these cases, additional X-ray topograms provide with others

Pro j ect direction a higher security for the recognition. Such X-ray topograms can be created fully automatically using the method presented above. For this purpose, once the first topogram has been recorded, a further reflex on the associated zone circle (cf. FIGS. 6 and 7) must be used for the image by rotating about the already adjusted reference direction

/Expectations

REPLACEMENT SHEET (REGE 26)

Claims

Expectations

1. A method for the X-ray identification of cut diamonds, in particular for determining whether a fully cut diamond is identical to or different from a known fully cut diamond, in which

using a goniometer with five circles (29, Ω, X,

Σ, Δ) with monochromatic X-rays in an X-ray diffraction adjustment routine

The table facet of the diamond relative to the diamond lattice is determined by two crystallographic pole angles (ρ, φ), from which an orientation of the diamond is determined,

subsequently, with this orientation, at least one X-ray topographical transmission image of the internal defects of the diamond is made by means of monochromatic X-ray radiation at the Bragg angle, and the identity is checked by comparing the determined pole angles and X-ray topograms with pole angles obtained according to the same procedure and X-ray topograms of known cut diamonds.

2. The method according to claim 1, characterized in that several, in particular up to four X-ray topographic transmission recording of the internal defects of the diamond are made.

3. The method according to claim 1, characterized in that by means of the adjustment routine the normal to the table facet of the diamond <100> - or <110> direction is determined as the crystallographic reference direction and that the associated pole angle (ρ, ψ) with a high Accuracy can be measured.

REPLACEMENT BUTT (RULE 26)

4. The method according to any one of claims 1 to 3, characterized in that the adjustment routine is automatically carried out under computer control using software created for this purpose.

5. The method according to any one of claims 1 to 3, characterized in that knowing the position of the normals on the table facet in relation to the crystallographic base, given by the measured pole angle (ρ, <p), with fixed re ﬂ ection position and projection direction preferably only a single X-ray topogram (mother topogram) is recorded.

6. The method according to any one of claims 1 to 3, characterized in that with knowledge of a measured pole angle (p) between the normal to the table facet and the determined crystallographic reference direction, the number of possible re ﬂ ection positions and projection directions for the acquisition of X-ray topographs to a maximum of four is limited.

7. The method according to any one of claims 1 to 6, characterized in that the recorded (mother) X-ray topograms are recorded on microfilm or on other data carriers for an existing or new diamond certificate.

8. The method according to any one of claims 1 to 7, characterized in that a dif fractometric measurement of the pole angle (o, φ) and subsequently a single X-ray topographic control recording (daughter topogram) is carried out to identify a recovered diamond.

9. The method according to any one of claims 1 to 8, characterized in that for quickly finding mother topograms for checking the identity with the Daughter topogram (control recordings) the two

Polwinkel (ρ, Φ) are prepared in a computer-readable form.

/ Summary

REPLACEMENT BUTT (RULE 26)