World Of Gems Conference 2017 Proceedings Book

WORLD OF GEMS CONFERENCE V - 5 - SEPTEMBER 2017 T here is a rising number of near-colorless synthetics di- amonds detected in the market, which create concerns for the trade. Production of synthetic diamonds has dra- matically risen in China (from 18 million carats in 2006 to 4 billion carats in 2011) and India. Two methods of growth are currently competing: HPHT growth (which can use any of four different technologies, most with a Fe-Ni flux) or CVD (mostly microwave assisted). The recent advent of near-colorless melee size synthetic diamonds has raised the question of how to test large numbers of small stones. In China, some companies are designing methods dedicated to growing gem diamonds, either large or melee. In response to this new sit- uation, many have turned to special- ized melee sorting machines, which are expensive and difficult to evaluate. The problem arises mostly within near- colorless type IIa diamond gems, chemically pure diamonds, with no trace of nitrogen detected by infrared absorption spectroscopy. This type is normally identified by infrared spec- troscopy, which is man- or instrument- intensive. For the jeweler-gemologist, this can be identified by SWUV trans- parency: SWUV transparent diamonds are type IIa, or the even rarer type IaB (which behaves similarly to type IIa). A number of small instruments working on this principle have been developed with varying efficiency. It is not difficult to build a “home-made” device, such as a “Phosphoroscope.” Figure 1. This “tester” measures SWUV trans- parency satisfactorily. However, its two conclusions are erroneous: “dia- mond” means in fact that it is a non-SWUV transparent, prob- ably type Ia diamond, whereas “CVD/HPHT” means that it is a type IIa (or type IaB), and could be natural, synthetic or HPHT-treated. Then, amongst type IIas, one has to determine which are synthetic. In our perception, synthetics are not the biggest gemological challenge today. HPHT-treated near-colorless diamonds are far more difficult to detect for mainly two rea- sons: while there is an abundant production and literature on synthetic diamonds, both useful articles on HPHT- treated stones and the resulting gems are scarce. In our experience, published criteria are no longer effective for all cases met in the lab, and near-colorless HPHT-treated stones may be overlooked. Once type IIas have been sorted out, if one is lucky, simple observations may help. The stone might be laser-inscribed as synthetic or HPHT-treated, of course never for melee size gems. If the stones hold flux inclusions, they may be recognized with magnifica- tion, or result in attraction to a mag- net. Using a simple UV lamp, synthetics tend to luminesce stronger in shortwave than longwave ultraviolet, unlike naturals: this is ef- fective to pinpoint HPHT-grown color- less and yellows, but useless for generally inert CVD-grown ones. With the same set-up, short-wave UV phosphorescence has proven helpful for HPHT-grown near-colorless dia- monds of all sizes: synthetics do phosphoresce strong blue for a “long” time, while phosphorescing naturals are much rarer and have slightly different properties. It is inter- esting to note that recently-produced HPHT-treated CVD synthetics do phosphoresce as well. To identify synthetics, one of the most effective sorting methods is anomalous double refringence (ADR; incorrectly called “birefringence” in many papers) observed between crossed polarizers in a high-index immersion liquid. However, this method finds its limit with very small diameter stones, as then the optical pathlength is so small that no “birefrin- NEAR-COLORLESS SYNTHETIC DIAMONDS ARE NOT THAT HARD TO IDENTIFY, HPHT TREATED ONES ARE Emmanuel Fritsch, PhD COLORLESS DIAMOND ABSTRACT FIGuRE 1. Phosphoroscope.

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