ARE NEAR-COLORLESS SYNTHETIC DIAMONDS TRULY DIFFICULT TO IDENTIFY?

There is a rising number of near-colorless synthetics diamonds detected in the market, which create concerns for the trade. The recent advent of near-colorless melee size synthetic diamonds has raised the question of how to test large numbers of small stones. Many have turned to specialized sorting machines, which are expensive and difficult to evaluate.

The problem arises only within near-colorless type IIa diamond gems. This type is commonly identified by infrared spectroscopy, which is man- or instrument-intensive. For the jeweler-gemologist, this can be identified by UV transparency, which isolates type IIa and also the even rarer type IaB from other types.

Then amongst type IIas, one has to determine which are synthetic. One of the most effective sorting methods is anomalous double refringence (ADR) observing 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 birefringence is perceived. Another approach is luminescence, based on observation of the whole stone; typically synthetics luminesce stronger in shortwave than longwave ultraviolet: this is effective to pinpoint HPHT-grown colorless and yellows, but useless for generally inert CVD-grown ones. Then the observation of luminescence zoning is usually conclusive. It necessitates expensive specialized instruments and also, a good knowledge of the very varied forms of natural diamond growth to avoid mistaking a natural for a synthetic. Finally, the last resort is luminescence spectroscopy. this typically involves laser excitation with several lasers, at low temperature. Further, this implies a very thorough knowledge of natural and synthetic diamond emissions, which limits this approach to a small number of laboratories. In practice, for a small number of stones, the definitive interpretation of spectra may remain a matter of discussion.

Thus, the separation of natural from synthetic diamond mélé may appear very difficult. Yet, there are a limited number of simple devices which can help the jeweler-gemologist, even more the diamond professional. The last resort appears to be luminescence spectroscopy, but it is expensive and a great deal of interpretation is required. Automated instruments offer only limited help.