Examining a diamond

In order to test whether a diamond has had a layer a synthetic diamond deposited thereon, the diamond is radiated with ultraviolet radiation so as to form a pattern of beams of refracted and reflected radiation, the pattern of refracted and reflected radiation being observed on a screen behind the diamond.

BACKGROUND TO THE INVENTION
 The present invention relates to a method of and apparatus for testing
 whether a natural diamond has had a layer of synthetic diamond deposited
 thereon. This is of 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
 natural diamond which is then worked, for example, into a round brilliant
 cut. Alternatively, the synthetic diamond material coating may be
 deposited onto a fully fashioned brilliant stone after working of the
 stone. The thickness of the synthetic diamond material layer may be very
 thin (it could be in the range from 5 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 cutting of the diamond, to increase the weight of the
 finished product.
 However, the value of a diamond 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 synthesising 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 material 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 such as physical vapour deposition have been
 proposed. A diamond artificially enlarged by deposition of CVD or similar
 diamond material is referred to in this specification 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 diamond has had a layer of synthetic diamond
 deposited thereon is disclosed in British Patent Application No. 9401354.7
 published as GB2286251A. A plurality of parts of the diamond are
 irradiated with radiation substantially of wavelength substantially in the
 range 230 nm to 320 nm and the transmission of the irradiating radiation
 by the diamond is observed.
 The invention of GB 9401354.7 is based upon the observation that where
 different zones of a diamond show differences in their absorption of
 radiation substantially of wavelength substantially 230 nm to 320 nm, 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 230 nm to 320 nm, the diamond may be classified as almost
 certainly a wholly natural diamond.
 The intensity of radiation transmitted by the zones of the diamond may be
 investigated using an imaging 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
 apparatus for testing whether a diamond has had a layer of synthetic
 diamond deposited thereon, in which relatively simple imaging 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 put into 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 training in laboratory
 gemological analysis.
 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 beams of radiation due to refraction and
 reflection of the irradiating radiation, and observing the pattern of such
 beams of radiation substantially of wavelength substantially in the range
 230 nm to 320 nm
 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 9401354.7.
 It is known from documents such as U.S. Pat. No. 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.
 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 normally be of a type which
 strongly 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 present face of the diamond is
 irradiated. This allows a complete pattern of beams to be formed and
 observed.
 In principle, a single observation of the pattern of refracted and
 reflected beams of radiation could be sufficient to reveal the presence of
 a layer of synthetic diamond material. If, for example, a substantially
 symmetrical face of the diamond is exposed to the radiation and an
 asymmetric pattern of beams is obtained, the presence of layers 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 patterns
 obtained. Interpretation of the results will be discussed further below.
 It may be sufficient to test only a few faces (maybe only two) in order to
 detect a difference in the pattern of reflected and refracted beams.
 Preferably, however, a large 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 irradiating
 radiation may be focussed if necessary.
 The beam of irradiating radiation may be of size less than the presented
 face of the diamond but is preferably greater in size.
 In the invention, the pattern of reflected and refracted beams observed
 does not correspond to the image of the diamond. What is observed is the
 pattern produced where the reflected and refracted beams intercept a
 notional plane displaced from the diamond. A screen or scanning means may
 be placed at this notional plane. The scanning means may measure the
 intensity of light at each point on the notional plane to thereby record
 the pattern of reflected and refracted beams.
 Preferably, the pattern of reflected and refracted beams is observed by
 placing a screen a predetermined distance from the diamond so that the
 beams of reflected and refracted radiation impinge upon the screen, and
 detecting the pattern on the screen. Preferably an image of the pattern on
 the screen is formed.
 The screen may be movable and angularly adjustable with respect to the
 diamond.
 The screen is particularly preferably placed on the
 direction-of-irradiation side of the diamond, so that back-scattered
 reflected and refracted beams are observed. In this case, it is preferable
 that the irradiating radiation passes to the diamond through an aperture
 in the screen.
 The screen may comprise an ultraviolet sensitive fluorescent screen for
 revealing the pattern of beams produced. In this case, the screen may be
 observed by eye through an observing means having a filter for cutting out
 hazardous irradiating radiation.
 Alternatively, a camera may be used to observe the screen.
 The radiation observed could comprise a narrow band of wavelengths lying
 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 230 nm to 300 nm, being preferably below 290 nm. The
 radiation observed may comprise some radiation of wavelength falling
 outside the range 230 nm to 320 nm but such radiation is preferably 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 248 nm krypton
 fluoride 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 by a laser or by a wider band source having a filter).
 Alternatively, the diamond may be irradiated with radiation of a broader
 range of wavelengths, wavelength selective means such as a filter being
 provided between the diamond and the screen or imaging means to pass
 radiation of wavelength substantially 230 nm to 320 nm. If the diamond is
 irradiated with radiation substantially of wavelength substantially 230 nm
 to 320 nm, wavelength selective means may also be provided to exclude
 radiation produced by fluorescence excited by the incident ultraviolet
 radiation. Normally, however, the intensity of fluorescence is not strong
 enough to require filtering.
 When the irradiating 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 diamond is
 irradiated.
 The intensity of reflected beams from any given surface will depend in part
 upon the transmissivity of that surface and in part upon the angle of
 incidence of the radiation upon the surface. The intensity of refracted
 radiation beams will depend in part 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.
 CVD or other synthetic diamond material surface layers are commonly of a
 type that at least partially transmits the radiation, in particular type
 II diamond.
 Thus, where a face of a diamond is irradiated normally and substantially no
 refracted beams are produced other than the reflection normal to the face,
 it may be concluded that the face is probably natural diamond.
 Where a face is normally irradiated and pattern of weak reflected and
 refracted beams is observed, the presence of a thin layer of synthetic
 diamond is indicated.
 Where a face of a diamond is irradiated at a relatively large angle off the
 normal (referred to as "oblique irradiation"), and a relatively weak and
 simple pattern of reflected beams is produced, it may be concluded that
 the face irradiated comprises natural diamond. If, however, a pattern of
 relatively strong and complex reflected and refracted beams is observed,
 the presence of synthetic diamond material is suggested.
 Any suggestion of synthetic diamond material should be followed up with
 further testing, as the reflected and refracted beams may be due to
 natural diamond of a rare type.
 If a diamond is irradiated on a face which is substantially symmetrical,
 and a pattern which is grossly unsymmetrical (for example, light on one
 side, dark on the other) is produced, it may be concluded that the sides
 of the face of the diamond presented are of different composition.
 Because of the complex pattern of light paths within a brilliant-cut
 diamond, the two parts of a CVD/natural 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 patterns of reflected and
 refracted beams produced when a diamond is irradiated with the first
 mentioned radiation, the diamond may be irradiated with radiation which is
 substantially transmitted by all types of diamond, such as visible
 radiation, so that a reference pattern may be formed. This pattern may
 then be compared to a pattern obtained using the first mentioned
 radiation, preferably with the diamond in the same configuration.
 The reference pattern is expected to show relatively strong and complex
 patterns 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 irradiating the diamond with ultraviolet radiation, and
 a screen mounted a predetermined distance from the diamond so that the
 screen intercepts a pattern of beams of reflected and refracted radiation
 produced when a diamond is irradiated, and
 means for allowing the pattern of beams of radiation substantially of
 wavelength substantially in the range of 230 nm to 320 nm on the screen to
 be observed
 The apparatus according to the invention could be automated to
 automatically interpret and analyse images or readings produced. However,
 this is not preferred as a simple system in which the images are
 interpreted by the operator is practicable and cheaper.
 The invention will be further described by way of example only, with
 reference to the accompanying drawings, in which:

DETAILED DESCRIPTION OF THE DRAWINGS
 In the apparatus shown schematically as 1 in FIG. 1, a diamond 2 is
 irradiated with radiation of wavelength substantially in the range 230-320
 nm by a laser 3. The laser beam 4 is directed through a screen 5, through
 an aperture 6 provided in the middle thereof. When the beam of radiation 4
 is incident upon the diamond 2, a pattern of beams of reflected and
 refracted radiation may be produced. The pattern produced in the
 back-scattered direction is studied in the embodiment shown in FIG. 1. The
 screen 5 is movable and angularly adjustable. The pattern is studied by
 arranging the screen 5 at a distance from the diamond 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.times.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 screen 5.
 The screen 5 is a UV fluorescent screen, which generates spots of visible
 light where ultraviolet radiation of wavelength 230-320 nm is incident
 upon it. The observing means 7 may comprise a suitable optical device with
 a filter for filtering out radiation of ultraviolet 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 excluding external radiation which may confuse the
 pattern on the screen and for containing the dangerous UV radiation. The
 observing means 7 may be mounted at a suitable position within the walls
 for the light-tight box so that an observer can see the pattern on the
 screen 5.
 In order to provide a reference pattern, a laser 8 producing light of a
 visible wavelength is provided. A beam splitter 9 is provided in the path
 of beam 4 so that the visible radiation from laser 8 may be directed down
 the path of the irradiating radiation 4 from laser 3. Preferably, lasers 3
 and 8 are used in alternation so that the different patterns produced by
 the different types of radiation may be compared.
 In FIGS. 2a to 2f, the results of irradiation of a diamond according to the
 invention are shown.
 Three cases were studied:
 a. A diamond which is a CVD/natural diamond doublet, with the synthetic
 part on the culet of the diamond,
 b. A CVD/natural diamond doublet in which the synthetic diamond is formed
 on the table of the diamond,
 c. A completely natural diamond
 In each case, the diamond is a cut diamond having a brilliant cut, being
 the type of cut which will be most frequently encountered. The technique
 is, however, applicable to all diamond cuts, including fancy cuts,
 although a more complex and careful interpretation of the returned pattern
 may be required for fancy cuts.
 The diamond is irradiated using the three steps:
 1. irradiation of the table in a normal direction using ultraviolet
 radiation of wavelength substantially in the range 230-320 nm,
 2. normal irradiation of the table using visible radiation, and
 3. irradiation of the culet using ultraviolet radiation substantially of
 wavelength falling in the range substantially 230-320 nm.
 The above-mentioned three types of diamond can be distinguished by the
 different patterns of reflected and refracted radiation that they produce.
 In FIGS. 2a-2f, sport of high intensity 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 FIGS. 2a-2c, the results of steps 1 and 2 are shown on a single screen
 for comparison, though in practice they would be separate.
 FIG. 2a shows the results of steps 1 and 2 with a diamond (a).
 The pattern on the screen in step 1 is observed to comprise a single high
 intensity spot 10 produced by normal reflection of the irradiating
 radiation.
 In step 2, a complex relatively intense pattern of spots 11 is observed.
 FIG. 2b shows the results of steps 1 and 2 with diamond (b) In step 1, a
 pattern of reflected and refracted beams 12 of relatively low intensity is
 observed. In step 2, a pattern of reflected and refracted beams of
 relatively high intensity is produced. The patterns are different, as the
 refractive index of diamond at the ultraviolet wavelengths observed is
 different to the refractive index of visible radiation.
 FIG. 2c shows the results of steps 1 and 2 with diamond (c). In step 1 a
 single relatively high intensity spot 14 is produced by normally reflected
 radiation only. In step 2, a relatively intense and complex pattern of
 reflected and refracted beams 15 is produced. The patterns observed in
 FIG. 2c are similar to those shown in FIG. 2a.
 FIG. 2d shows the results of step 3 with the diamond (a). A relatively
 complex pattern of strong reflected and refracted beams 17 is produced,
 together with a strong beam 16 due to radiation reflected normally from
 the culet (assuming that there is a culet facet).
 FIG. 2e shows the results of step 3 with diamond (b). A relatively weak
 simple pattern of reflected beams 18 is produced due to reflection off the
 cut surfaces around the culet.
 FIG. 2f shows the results of step 3 with diamond (c). A simple pattern of
 relatively weak reflected beams 19 is produced.
 In the apparatus shown in FIG. 1, the ultraviolet laser may comprise a 248
 nm krypton fluoride excimer laser from Potomac lasers. The laser 8 may
 comprise a 635 nm laser diode or 633 nm HeNe laser from Vector
 Technology/Melles Griot. The beam splitter 9 is manufactured by Spindler
 and Hoyer and the ultraviolet 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.