Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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E~MINTNG A DIAMOND
Back round to the Invention
The present invention relate5 to a method of and apparatus for testing whether anatural diamond has had a la,ver of synthetic diamond deposited thereon. This isof particular importance in testing whether the diamond is wholly natural or
whether any part of it comprises CVD diamond material and also in locating such
material if present.
Synthetic diamond material may be deposited on an uncut or part processed
naturai diamond which is then worked~ for e~cample, into a round brilliant cut.
Alternativel,v. the synthetic diamond material coating ma~ be deposited onto a
fully fashioned brilliant stone after workin_ of the stone. The thickness of thesvnthetic diamond material laver may be ~er~ thin (it could be in the range from5 microns to 10 microns) but the present invention may also be used to detect
thicker layers.
The value of a diamond is in part dependent upon its weight. Accordingly,
synthetic diamond material may be deposited onto natural gem diamonds. before
or after cuttin_ of the diarnond~ to increase the weight of the finished product.
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However, the value of a diam ond also resides in its qualities of authenticity and
uniqueness and in the fact that it is an entirely natural (ie mined) product. Thus,
a diamond that has not been enlarged by deposition of synthetic diamond
material has a value over a diamond which has.
Over the years, a number of methods of synth~ising diamond material have
been developed. One of these methods is the chemical vapour deposition
(CVD) technique, which is a low pressure technique involving deposition of
synthetic diamond (referred to as CVD diamond m~t~i~l in this specification)
onto a substrate from a gas. CVD is the most likely way in which synthetic
diamond will be deposited on a diamond, although alternative techniques sùch
as physical vapour deposition have been proposed. A diamond artificially
enlarged by deposition of CVD or similar diamond material is referred to in thisspecification as a "CVD/natural diamond doublet".
CVD diamond material may be deposited on a non-diamond or diamond
substrate. In the latter case, the CVD diamond material can replicate the
structure of the diamond substrate (referred to as "homoepitaxial growth"). The
CVD/natural diamond doublet produced can be identical in appearance, density
and other common physical properties to an entirely natural stone and there may
be a problem in identifying such a CVD/natural diamond doublet.
A method of testing whether a ~ mont1 has had a layer of synthetic diamond
deposited thereon is disclosed in GB 2 286 251A. A plurality of parts of the
diamond are irradiated with radiation substantially of
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wavelength substantially in the range 230nm to 320nm and the tr~nxmi.~.~ion of
the irr~ ting radiation by the diamond is observed.
The invention of GB 2 286 25 lA is based upon the observation that where -
different zones of a diamond show differences in their absorption of radiation
substantially of wavelength substantially 230nm to 320mn, it may be concluded
that the diamond in question has a layer of synthetic diamond deposited thereon.It is further observed that if all zones of a diamond strongly absorb radiation
substantially of wavelength substantially 230nm to 320nm, the diamond may be
classified as ~lmost cert~inly a wholly natural diamond.
The intensity of radiation tr~n~mitte-l by the zones of the diamond may be
investigated using an im~ging apparatus or by placing the diamond in an
integrating sphere. Preferably, an image of the diamond is formed against a
dark or light background.
It is an object of the present invention to provide a method of and ay~ us for
testing whethOE a diamond has had a layer of synthetic diamond deposited
thereon, in which relatively simple im~ging apparatus is used and an expensive
integrating sphere is not required.
It is desired that the apparatus should be simple and inexpensive and may be putinto operation by a person with relatively little training. The method and
apparatus should be capable of being operated reliably and consistently by a
practised jeweller who has no tr~ining in laboratory gemological analysis.
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The Invention
The present invention provides a method of testing whether a diamond has had
a layer of synthetic diamond deposited thereon, comprising: -
directing a beam of ultraviolet radiation towards a face of a diamond, so
as to form a pattern of spots due to beams of radiation caused by refraction andreflection of the irra~i~ting radiation, and observing the pattern of such spotsdue to beams of radiation substantially of wavelength substantially in the range230nm to 320nm.
The present invention also provides apparatus for testing whether a diamond has
had a layer of synthetic diamond deposited thereon, comprising:
means for irr~ ting the diamond with ultraviolet radiation;
a screen mounted at a pre~:let~-mine(l distance from the diamond so that
the screen intercepts a pattern of beams of reflected and refracted radiation
produced when a diamond is irr~ te-1; and
means for allowing the pattern of spots due to beams of radiation
substantially of wavelength substantially in the range 230nm to 320nm on the
screen to be observed.
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Document#:171766 4a
The present invention uses the same principles of absorption of certain wavelengths of ultra-
violet radiation by certain types of diamond as used in GB 2 ~86 251A.
It is known from documents such as US 3 740 142 and US 3 947 120 that where light is
directed towards a cut gemstone, a pattern of spots of reflected and refracted radiation may
be produced which is characteristic of each gemstone.
The present inventors have discovered that the different interaction of different types of
diamond with ultraviolet radiation of the waveband in question can affect the pattern of
spots obtained and help to identify superficial synthetic diamond layers.
In simple terms, substantial differences in the complexity and intensity of beams produced
by different parts of the diamond (allowing for the shape of the diamond) indicate the
presence of synthetic layers on the diamond.
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In detail, the invention is based upon the observation that the majority of natural
diamonds are classified as type IaA or IaAB and very strongly absorb
ultraviolet radiation of wavelength shorter than approximately 320 nm, whereas
a synthetic diamond layer will norrnally be of a type which strongl~- absorbs
ultraviolet radiation of wavelength shorter than approximately 230 nm, in
particular type II diamond. Thus natural diamond is generally expected to give
weak or unobservable reflected and refracted beams with radiation of
wavelength shorter than 320 nm.
A synthetic diamond layer is generally expected to give a complex pattern of
reflected and refracted beams. Any diamonds which give results suggesting the
presence of a synthetic layer should be referred for further testing
Preferably, substantially the whole of the presented face of the diamond is
irradiated. This allows a complete pattem of beams to be formed and observed.
In principle, a single observation of the pattem of refracted and ref~ected beams
of radiation could be sufficient to reveal the presence of a layer of svnthetic
diamond material. If, for e~cample. a substantially symmetrical face of the
diamond is exposed to the radiation and an asymmetric pattern of beams is
obtained, the presence of lavers of synthetic diamond may be suspected.
However, it is preferable to direct the beam of radiation to the diamond from a
number of directions in succession and to compare the pattems obtained.
Inlel ~,el~Lion of the results will be discussed further below.
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It may be sufficient to test only a fe-v faces (maybe only t~ho) in order to detect
a differenc~ in the pattem of reflected and refracted beams. Preferably,
however, a lar_e number of faces are irradiated in succession.
The diamond may be irradiated with suitable radiation (as discussed below) by
exposing it to radiation from a suitable source The irra~iating radiation may befocussed if necessary.
The beam of irr~di~ting radiation may be of size less than the presented face ofthe diamond but is preferably greater in size.
In the invention, the pattem of reflected and refracted beams observed does not
correspond to the image of the diamond. What is observed is the pattern
produced ~here the reflected and refracted beams intercept a notional plane
displaced trom the diamond. A screen or sr~nnino means may be placed at this
notional plane. The sr~nnino means may measure the intensity of li~ht at each
point on the notional plane to thereby record the partern of reflected and
refracted beams.
Preferablv the pattem of reflected and refracted be2ms is observed by placin~ a
screen a predeterrnined distance from the diarnond so that the beams of
reflected and refracted radiation impinoe upon the s,reen, and detectino. the
pattern on the screen. Preferably an image of the pattem on the screen is
fommed.
The screen may be movable and angularly adjustabie with respect to the
diamond.
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The screen is particularly preferably placed on the direction-of-irradiation side
of the diamond, so that bacL;-scattered reflected and refracted beams are
observed. In this case, it is preferable that the irra~ ting radiation passes to the
diarnond through an aperture in the screen.
The screen may comprise an ultra~-iolet sensitive fluorescent screen for
revealing the pattern of beams produced. In this case, the screen may be
observed by eye through an obser~ing means having a filter for cutting out
hazardous irr~ ring radiation.
Altematively, a camera may be used to observe the screen.
The radiation observed could comprise a narrow band of wavelengths Iying
substantially in the above mentioned range~ a number of such narrow bands or it
could be a relatively broad band. Optionally, it falls substantially in the range
230nm to 300nm. being preferably belou 290nm. The radiation observed may
comprise some radiation of wavelength falling outside the range 230 nm to 320
nm but such radiation is preferabl~ of sufficiently low intensity to avoid
confusing the beams observed at the wavelength of interest.
The radiation may be generated by a suitable laser. e.g. a 24Bnm krypton
fiuoride excimer laser.
In order to observe radiation substantially of wavelength substantially 230 nm
to 320 nm, the diamond may be irradiated only with such radiation (produced
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by a laser or by a ~ider band source having a filter). Alternatively. the
diamond may be irradiated with radiation of a broader range of u-avelengths,
wavelen_th selecti- e means such as a filter being provided between the
diamond and the s~een or im~gino means to pass radiation of wavelength
substantially 230 mn to 3~0 nm. If the diamond is irradiated with radiation
substantially of wavelength substantially 230 nm to 320 r~n, wavelength
selective means may also be provided to exclude radiation produced by
fluorescence excited by the incident ultraviolet radiation. ~ormally, however,
the intensity of fluorescence is not strong enough to require filtering.
When the irr~ ting radiation is incident on a zone of the diamond, it will
generally be strongly absorbed or partially transmitted. The radiation
transmitted by a zone of the diamond will be refracted inside the diamond and
some transmitted radiation may be observed leaving the surface of the diamond.
Thus~ a pattern of beams of reflected and refracted radiation will be produced
when a face of a diarnond is irratli~tt~l
The intensitv of retlected beams from any ~iven surface ~ill depend in part
upon the ~ransmissivity of that surface and in part upon the angle of incidence
of the radiation upc!n the surface. The intensitv of refracted radiation beams
will depend in par~ upon the transmissivity of the diamond material of a part
observed and in part on its thickness.
Natural diamond usually has such a high absorption coefficient at the
wavelengths in question that incident radiation is almost totally absorbed.
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C~ or other synthetic diamond material ,urface layers are commonly of a type
that at least partially transmits the radiati~;l. in particular type II diamond
Thus, where a face of a diamond is irradiated nommally and substantially no
re~racted beams are produced other than the reflection normal to the face, it
mav be concluded that the face is probably natural diamond
U~ere a face is normally irradiated and a pattem of weak reflected and
refracted beams is observed. the presence of a thin layer of synthetic diamond is
indicated
Ullere a face of a diamond is irradiated aI a relatively large angle off the normal
(referred to as "oblique irradiation"), and a relatively weak and simple patternof reflected beams is produced, it may be .oncluded that the face irradiated
comprises natural diamond If, however. a pattem of relatively strong and
complex reflected and refracted beams is observed, the presence of svnthetic
diamond material is suggested
Anv suggestion of synthetic diamond malerial should be followed up withfurther testing. as the reflected and refracIed beams may be due to natural
diamond of a rare type
If a diamond is irradiated on a face whic~. is substantially symmetrical. and a
partern which is grossly unsymmetrical I ~or example, light on one side. dark onthe other) is produced, it may be concluded that the sides of the face of the
diamond presented are of different composition
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Because of the complex partem of li .ht paths within a brilliant-cut diarnond, the
two parts of a CVD/naturai diamond doublet may not be immediately apparent.
It may be necessary to manipulate a CVD/natural diamond doublet while it is
being viewed, in order to clearly see the two parts of the diamond.
In order to assist in the interpretation of the pattems of reflected and refracted
beams produced when a diamond is irradiated with the first mentioned
radiation, the diamond ma~- be irradiated with radiation which is substantially
transmitted by all types of diamond, such as visible radiation, so that a reference
pattem may be fommed. This pattem may then be compared to a pattern
obtained using the first mer.tioned radiation, preferably with the diamond in the
same configuration.
The reference pattern is e~;pected to show relatively strong and comple~
pattems of reflected and refracted radiation for all types of diamond.
The present invention further provides apparatus for testing whether a diamond
has had a layer of synthetic diamond deposited thereon. comprising means for
irra~ ting the diamond v.itn ultraviolet radiation. and
a screen mounted a predetermined distance from the diamond so that the screen
intercepts a pattem of beams of reflected and refracted radiation produced when
a diamond is irradi,.t~rl, and
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means for allowing the pattern of beams of radiation substantially of
wavelength substantially in the range 230nm to 320nm on the screen to be
observed.
The apparatus according to the invention could be automated to automatically
interpret and analyse irnages or readings produced. However, this is not
preferred, as a simple system in which the images are interpreted by the
operator is practicable and cheapOE.
The invention will be further described by way of example only, with reference
to the accompanying drawings, in which: -
Brief Description of the Drawin~s
Figure 1 is a scht-m~tic illustration of apparatus according to the invention; and
Figures 2a - 2f are s~h~m~tic illustrations of patterns of reflected and refracted
beams produced according to the present invention when various tli~mnnl1~ are
irradiated with ultraviolet or visible radiation, the diamonds being illustratedschematically.
Detailed Description of the Drawin~s
In the apparatus shown s~.h~m~tically as 1 in figure 1, a diamond 2 is irradiated
with radiation of wavelength substantially in the range 230-320nm by a laser 3.
The laser beam 4 is directed through a screen 5, through an aperture 6 provided
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diamond 2, a pattern of beams cf reflected and refracted radiation may be
produced. The pattern produced in the back-scattered direction is studied in theembodiment shown in figure I The screen 5 is movable and an.~ularly
adjustable. The pattern is studied by arranging the screen 5 at a distance from
the diarnond 2 such that substantially all the beams of reflected and refracted
radiation are intercepted by the screen. Typically, for a screen of size 100 mm
x 100 mm, the distance between the diamond and the screen is circa
60 mm.
An observing means 7 is provided for observing the pattern of reflected and
refracted beams formed on the ~creen 5.
The screen 5 is a W fluorescent screen. which generates SpoB of visible light
where ultraviolet radiation of v.-avelength ''30-320nm is incident upon it. The
observing means 7 may compri~e a suitable optical device with a filter for
filterin out radiation of ultra--~olet wavelengths~ which can be dangerous to the
eye.
The whole apparatus 1. except for the observing means 7 may be enclosed in a
light-tight box, for e~ccludin_ e~ ernal radiation which may confuse the patternon the screen and for cont~inin~ the dan_erous UV radiation. The obser~ing
means 7 may be mounted at a suitable position within the walls of the li~ht-ti_ht
box so that an observer can see the pattern on the screen 5.
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In order to provide a reference pattern, a laser 8 producing light of a visible
wavelength is pro~ ided. A beam splitter 9 is provided in the path of beam 4 so
that the v isible rahiation from laser 8 may be directed do~n the path of the
irradiating radiation 4 from laser 3. Preferably, lasers 3 and 8 are used in
alternation so tha~ the different patterns produced by the different types of
radiation may be compared.
In figures 2a to 2~. the results of irradiation of a diamond according to the
invention are shonn.
Three cases were jtudied:
a. A diamond ~hich is a CVD/natural diamond doublet, with the synthetic
part on the culet of the diamond,
b. A CVD/na.~ral diamond doublet in which the svnthetic diamond is
formed on the ta~ie of the diamond,
c. A complete!- natural diamond
In each case. the iiamond is a cut diamond having a brilliant cut, being the type
of cut which will be most frequently encountered. The teehnique is, however,
applicable to all diamond cuts, including fancy cuts, although a more complex
and careful inL~ alion of the returned pattern may be required for fancy cuts.
The diamond is irradiated using the three steps:
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irradiation of the table in a normal direction using ultraviolet radiation of
~-avelength subst~nti~lly in the range 233- 320nm,
'. normal irradiation of the table using visible radiation, and
,. irradiation of the culet using ultra--iolet radiation substantially of
~-avelength falling in the range substantially '~30-320nm.
The above-mentioned three types of diamond can be distinguiched by the
different patterns of reflected and refracted radiation that they produce.
In figures 2a - 2f, spots of high intensit~ are shown as a solid black dot, spots of
medium intensity are shown as short complete lines and spots of low intensity
are shown as short, dotted lines.
In figures 2a - ''c, the results of steps 1 2nd 2 are shown on a single screen for
comparison. though in practice thev ~ol;ld be separate.
Figure 2a shows the results of steps I a; d 2 with diamond (a).
The pattern on the screen in step I is o~,en, ed to comprise a single high
intensity spot 10 produced by normal reIlection of the irr~ ting radiation.
In step 2, a complex relatively intense pattern of spots 11 is observed.
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Figure 2b shov.-s the re,-~lts of steps I and 2 with diamond (b) In step 1, a
pattern of reflected anc refracted beams 12 of relatively low intensity is
observed. In step 2, a ~attern of reflected and refracted beams of relatively high
intensity is produced. The patterns are different, as the refractive index of
diamond at the ultravic et wavelengths observed is different to the refractive
index of visible radiati~n.
Figure 2c shows the re~lts of steps 1 and 2 with diamond (c). In step 1 a singlerelatively high intensir~ spot 14 is produced by normally reflected radiation
only. In step 2. a relati;-ely intense and complex pattern of reflected and
refracted beams 15 is ~oduced. The patterns observed in figure 2c are similar
to those sho~n in figu.-e 2a.
Figure 2d shows the re~llts of step 3 with the diamond (a). A relatively
comple~c pattern of strc-~, reflected and refracted beams 17 is Froduced,
together with a stron_ eam 16 due to radiation reflected normally from the
culet (assuming that th~re is a culet facet).
Figure 2e shows the re~lts of step 3 with diamond (b). A relaIively weak
simple pattern of refle. -d beams 18 is produced due to reflection off the cut
surfaces around the CUi_l.
Figure 2f shows the re~_lts of step 3 with diamond (c). A simp!e pattern of
relatively weak reflect i beams 19 is produced.
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In the apparatus shown in figure 1, the ultraviolet laser may comprise a 248nm
krypton fluoride excimer laser from Potomac lasers. The laser 8 may comprise
a 635 nm laser diode or 633nm HeNe laser from Vector Technology/Melles
Griot. The beam splitter 9 is manufactured by Spindler and Hoyer and the
ultra~, iolet sensitive fluorescent screen is supplied by Levy-Hill Ltd. If a
camera is used to observe the screen 5, it may be a CCD camera coupled to a
computer for analysing the spot pattern produced.
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