Synthetic Diamonds: Improved Quality and Identification Challenges
Colourless synthetic diamonds produced by Chemical Vapour Deposition (CVD) are becoming more common in the market. CVD-processed stones are very different from the natural diamonds produced by geological forces. They are also vastly different from the High Pressure, High Temperature (HPHT) synthetics that have been manufactured since the mid-1950s.
CVD synthetic diamonds began appearing in the gem diamond market about a decade ago. The process involves introducing a gas, such as methane, into a vacuum chamber, then activating and breaking down the molecules of the gas with microwaves. This causes the carbon atoms to accumulate on a substrate (a small platform containing a flat diamond seed crystal, usually an HPHT synthetic), similar to the way in which snowflakes accumulate in a snowfall.
The HPHT method, first used to successfully grow diamonds by General Electric in 1954, essentially mimics the natural process - carbon is crystallised as diamond through intense heat and pressure deep within the Earth. The HPHT process is very costly, given the energy and equipment required, and produces diamonds with mainly yellowish or brownish yellow colours.
The CVD method is much less costly, because it works at moderate temperatures and low pressure, which requires smaller and less expensive equipment. Because the vacuum chamber contains only carbon and some hydrogen, colourless crystals can be grown. If nitrogen or boron is introduced into the chamber, yellow or blue synthetic diamond crystals can be produced.
At first, the creators of these CVD diamonds thought that they would be able to produce diamonds quickly and cheaply. However, because the quality was inconsistent and the typical brown colours were less than desirable, those expectations were not met.
In the past decade, CVD producers have found that changing the gases in the growth chamber and using a purer Type II synthetic diamond as a seed crystal can improve the colour of the finished synthetic diamond and speed the growth rates. Since many early CVDs had a brownish colour, producers also found that treating the material at high temperatures and pressures can remove the brown colouration to make the crystals colourless. This treatment step masked some of the signature features of these synthetic diamonds, which resulted in their identification becoming more difficult.
The Gemesis Corporation of Sarasota, Florida, which produced its earlier synthetic diamonds with the HPHT-process, began to market colourless CVD synthetic diamonds in March 2012. Several other companies also sell CVDs.
GIA researchers purchased 16 CVD diamonds from Gemesis, 15 of them cut into round brilliants, ranging from 0.24 carats to 0.86 carats. The majority had very high colours, ranging from F to G on the GIA 4Cs scale. Three were I-J. The largest, at 0.90 carats, was a rectangle-cut graded L, since it showed a slight yellowish cast.
The researchers, led by Dr Wuyi Wang, GIA’s director of research and development, subjected these synthetic diamonds to an extensive battery of tests. The tests included several sophisticated types of spectral analysis to obtain the telltale “signature” of samples created by the CVD process. By using advanced instrumentation, scientists can read the diamonds’ spectral signature to determine their composition, or whether they originated in nature or a laboratory. The GIA researchers also conducted standard gemmological tests, including checking for features known to be diagnostic for other synthetic diamonds, such as graining patterns and ultraviolet fluorescence reactions. Diamonds have a grain, like wood, that is a result of the way in which they crystallise, but because they grow differently in nature compared to a lab, the grain patterns of synthetic diamonds are different.
The GIA tests found that the quality of colourless to near-colourless CVD-grown diamonds has improved significantly in the decade since they were introduced. In the past, CVD synthetics displayed graining patterns not found in natural diamonds, as well as distinctive fluorescence reactions. These samples showed that distinguishing such stones from natural diamonds requires the use of advanced spectroscopic techniques. In addition, it was apparent that the synthetics were treated under high heat to remove or obscure some of their tell-tale features, such as graining, which, after treatment, is very difficult to see.
Despite the difficulty in spotting these stones by conventional means, GIA researchers did find unique spectroscopic signatures. They did so by subjecting photoluminescence and ultraviolet fluorescence reactions to the DiamondView™, a sophisticated instrument used by gemmological labs to detect synthetic diamonds.
GIA continues to make synthetic diamonds a top research priority so that it can identify and report on the latest developments in these lab-created stones.