GEM QUALITY CVD SYNTHETIC DIAMOND

INTRODUCTION
Low pressure chemical vapour deposition CVD growth of diamond became a reality following a recommendation by Bridgeman in 1955, and developmental work by Derjaguin & Spitsyn in Russia that resulted in the growth of synthetic diamond by this method in 1968. By 1988, Angus and his co-workers at Case Western University had deposited synthesised diamond on diamond powder at a temperature of 1050 °C and less than a thousandth of an atmosphere pressure.

METHOD OF SYNTHESIS
In the chemical vapour deposition (CVD) process for synthesising diamond, a carbon containing gas, such as methane (CH₄), or propane (C₃H₈), is decomposed by heat (generated by microwave energy, radio frequency, tungsten filament, oxyhydrogen torch, or plasma torch) to produce a carbon-rich plasma of charged particles. The resulting carbon vapour, which is mixed with excess hydrogen to prevent oxidation of the carbon and maintain the tetrahedral arrangement of the carbon’s bonds, is then carried into an evacuated reaction chamber at a temperature of 600-1,500º C (depending on the pressure and the hydrocarbon). Under appropriate conditions, the carbon atoms will deposit onto a suitable substrate (including diamond) as diamond. The initial deposits of this man-made diamond were termed DLC (diamond-like carbon) for the coating formed on the substrate consisted of layer of diamond microcrystals of similar but somewhat fibrous alignment. Subsequently, diamond films of some thickness have been synthesises by simultaneously depositing films on two (?diamond) substrates separated by a specific distance—with deposition continuing until the diamond films merge to form a single layer.

Recent history
For many decades, major synthesisers of diamond for industrial purposes, such as GE, De Beers’ Element Six, and Sumitomo, have been refining CVD technology mainly for industrial purposes. Also, research institutes throughout the world, such as Paris University 13’s LIMHP-CNRS, have developed and are continuously refining CVD technology with the aim of synthesising films of monocrystalline diamond of 1-2 mm or more thickness of that have commercially acceptable colours such as colourless to near colourless or blue, rather than the brown colours yielded by CVD diamond that are grown  commonly in a nitrogen atmosphere and require additional HPHT treatment to yield near colourless diamond.

In late 2003, Bryant Linares of Apollo Diamonds indicated that since 2000 his company had been producing gem quality synthetic single diamond films of sufficient thickness to allow the faceting of small (0.25 ct plus) diamonds. Rough, capable of faceting up to 1 ct stones, was predicted to become available the near future. These thick, single diamond films are being grown by still-secret CVD technology. Linares claimed that their CVD diamonds were more perfect than natural diamonds, with inclusions in the VS range, and colours that can range from brown to HPHT induced colourless to near colourless, blue, pink, and black. These diamonds are essentially type II diamond (blues being type IIb). Apollo Diamonds is planning to commercially market their CVD diamonds.

According to Apollo Diamond’s website at http://www.apollodiamond.com, Apollo Diamonds insert multiple diamond seeds, each the size of a shirt button, into the growth chamber of their CVD facility. Once grown, each single crystal synthetic diamond is separated from its seed, and if of appropriate quality faceted. Each Apollo Diamond™ will be laser inscribed with a serial number and certified as to its true nature. It is planned that the Apollo Diamond Webstore will be opened to the public during 2006

Towards the end of 2005, a report from the Carnegie Institution's Geophysical Laboratory in the USA claimed that it was now possible to synthesise a colourless 10-carat diamond in a matter of hours at growth rates of 100 micrometers per hour. To achieve this increase in the size of synthesised crystal, these researchers used the CVD process to grow gem-quality diamonds sequentially on the 6 faces of a plate of a diamond substrate.

The challenge
It is a fact that CVD synthesised gem quality brownish, colourless to near colourless (type 11a), and blue (type 11b) diamonds will soon enter the commercial market. Their identification will pose some challenges for gemmologists who do not have access to advanced gem testing instruments such as De Beers DiamondView™, or photoluminescence spectroscopy.

However, the trade gemmologist can use certain clues to refer a suspect diamond on for further testing. These clues include:

• the small size of the diamond (<0.50 ct).
• the SWUV transparency of type 11 diamond.
• a brownish colour (commonly), but the diamond could be colourless to near colourless or bluish.
• inclusions of black particulate non-diamond carbon.
• orange fluorescence to ultraviolet light of variable intensity (SWUV>LWUV).
• cross hatched bands of low-order interference strain birefringence when examined between crossed polars through the (100) face of the diamond.

When submitted to an appropriately equipped gem testing laboratory, CVD synthetic diamond will be detected by a combination of the following features:

• strong orange red fluorescence (due to the presence of N-V centres) when the diamond is examined with the high energy SWUV DiamondView™—an instrument that produces a SWUV tomogram of the diamond’s growth history; and,
• advanced spectroscopic features that include:

• the characteristic absorptions in the infra red between 2.6 and 7.2 µm (3800 to 1380 cm^-1) of type 11 diamond, with an additional peak at 1244 cm^-1 due to the presence of trace amounts of isolated nitrogen.

• strong photoluminescent peaks to the green 524.5 nm laser at 575 and 637 nm due to the presence of (N-V)⁰ and (N-V)^- centres.

• absorption due to hydrogen at 3123 cm^-1.

• a possible absorption due to silicon at 737 nm.

Conclusion
CVD diamonds may be seen by some non laboratory based gemmologists as a threat; but it is reassuring that gemmological researchers have found an answer to the question … is this diamond a CVD synthetic diamond or not.

Further reading: 

• De Gryse, O. & De Corte, K. (2005) Made in Europe: Synthetic CVD diamond.Antwerp Facets Magazine. 2(22), 24-51.

• Wang. W., Moses, T., Linares, R.C., Shigley, J.E., Hall, M. & Butler, J.E. (2003) Gem-quality synthetic diamonds grown by a Chemical Vapour Deposition (CVD) method. Gems & Gemology. 39(4), 268-283.

• Wang. W., Tallaire, A., Hail, M.S., Moses, T.M., Achard, J., Sussman, R.s. & Gicquel, A. (2005). Experimental CVD synthetic diamonds from LIMHP-CNES, France. Gems & Gemology. 41(3), 234-245.