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From lecture meetings of the Gemmological Society of Japan in 2004 2004.08.30
Update of Synthetic Diamond Identification
Gemmological Association of All Japan Research Laboratory
Hiroshi KITAWAKIiFGA, CGJj, Ahmadjan ABDURIYMiPh.D.-Minj, Makoto OKANOiCGJj
This report summarises a general lecture presented at the meeting of the Gemmological Society (Japan) in 2004.

Introduction
  HPHT (high pressure / high temperature) treated synthetic diamonds have started appearing on the gemstone market since 1990fs. Most of these stones are typeIb yellow stones under 2 carats in weight, but some are changed to greenish-yellow typeIa by HPHT process. A few of colourless typeIIa and blue typeIIb also exist. Some stones may be irradiated and heated to change the colour into pink to red or purple these days.
  Techniques from Russia have traditionally been used to synthesise gem-quality diamonds, but in recent years gGemesish of the United States is mass-producing yellow diamonds with its proprietary technique that was developed out of Russian technique, while Chatham is distributing diamonds produced by the technique that is different from the Russian method. Chatham was especially proactive in promoting its products at the Tucson Gem & Mineral Show, and it has wide range of line up in pink, blue or yellow colour (Photo-1).
Photo-1 Chatham created synthetic diamonds in various colours.

Identification of Synthetic Diamonds
| Basic Theory -
  Several methods known to synthesise diamonds are HPHT method, CVD method and shock method. Among them, HPHT method can produce diamonds for gem-use. Apollo Diamond Inc. released its production and sales of diamond by CVD method, but their mass production has not been reported yet. It may be difficult for them to compete with natural diamonds in price at the present stage, but CVD diamonds are to be watched hereafter.
  In HPHT method, metal solvents such as Fe, Ni or Co are used, because the main composition element C (carbon) is easy to dissolve in these metals. The type of metal used differ according to manufacturers or to the required colour of the resulted diamonds, but all of them eventually give crystal forms that are totally different from natural crystals. Also the synthesised diamonds often include those metals as inclusions, which can be a clue for identification.

Identification of Diamonds by Standard Methods
Inclusion

  Natural diamond is considered to be formed in the upper mantle, 150 to 200km under the ground. The co-existing minerals in the upper mantle are such as pyrope garnet, diopside, enstatite or olivine, which may be trapped in a diamond as inclusions (Photo-2).
Photo-2 Crystal inclusions in natural diamond
garnet
olivine diopside
Contrary to this, synthetic diamonds, as described before, often contain metal inclusions (Photo-3),
Photo-3 Metal inclusion in synthetic diamond
which are magnetic and if they are included abundantly in a diamond they may attract a magnet. Nitrogen- getter is used especially in synthesising colourless or blue diamonds, and the stones highly possibly include metals. In natural diamond, some metal inclusions such as chromite may be included but they show different form from the metals in synthetic diamonds and they are not magnetic.
Anomalous Double Refraction due to Distortion
@@Diamond belongs to cubic crystal system and is optically isotropic. However, most of natural diamonds show anomalous double refraction due to distortion under crossed polarising lights. The anomalous double refraction due to distortion observed in natural diamonds are basically divided into two types, one is caused during growth and the other by plastic deformation. The latter is especially used as an identifying feature for natural diamonds (Photo-4).
Photo-4 Anomalous double refraction due to distortion in natural typeIa diamond (left: distortion produced during the growing stage, right: distortion produced by plastic deformation).
A typical anomalous double refraction due to distortion by plastic deformation is gTatami-math of typeIIdiamond. This has been called so since a Japanese female researcher in 1970fs expressed the pattern of anomalous double refraction due to distortion observed in a typeII diamond as gTatami-mat likeh. Contrary to this, synthetic diamonds show this type of anomalous double refraction only along sectors and no gTatami-math pattern is seen.
Photo-5 Cross-pattern patches of fluorescence unique to synthetic diamonds.

Colour Zoning
  Natural diamond grows colour zoning within the stone according to the degree of included impurity elements (mainly nitrogen) during its growth. This colour zoning is generally parallel to the octahedral faces. Brown and pink colour zoning can also be formed by plastic deformation after the crystal growth (photo-6).
Photo-6 Brown grains in natural diamond.
They are parallel to octahedral faces and intersecting to each other, which are characteristic to natural diamond. In synthetic stones, distinct colour zoning in accordance with sector zoning is usually observed because the degree of included impurity elements differ in each sector. Thus colour zoning of cross/square combination is often observed in synthetic diamond, which can be an identifying feature (Photo-7).
Photo-7 Square colour zoning in synthetic diamond.
However, care should be taken that colour zoning of crossed pattern similar to those seen in synthetic stone may be observed in natural diamond called centre cross-type (Photo-8).
Photo-8 Centre-cross diamond (natural).
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