By James Shigley, Shane McClure, Shane Elen
Gemological Institute of America, GIA Gem Trade Laboratory
A company in North Carolina, known as C3 Inc., has announced plans to market a new, near-colorless diamond imitation material called synthetic moissanite to be used for gem purposes. Although it appears that this material is just now being released into the jewelry marketplace, press reports distributed by C3 representatives have caused questions and some concern among jewelers.
These questions have centered on what this new imitation material is, and how it can be quickly identified, since when tested with conventional thermal testers, synthetic moissanite reacts as if it were “diamond”, and not a “simulant” like cubic zirconia (CZ). Thus, a jeweler, relying upon this simple test alone, might misidentify this new imitation. C3 Inc. also plans to market a test instrument (the “Diamond Tester Model 590”) that distinguishes synthetic moissanite from diamond.
With cooperation of C3 representatives, GIA researchers were able to carry out a gemological study of synthetic moissanite. Complete results of this study have recently been published in the Winter 1997 issue of Gems & Gemology. To disseminate the results of this study more widely, the key means of identifying synthetic moissanite are summarized here.
Moissanite is the mineral name for silicon carbide. The natural mineral has been found as tiny grains in meteorites and some rocks. Silicon carbide is also manufactured on a large scale, and is an important industrial product widely used as an abrasive because of its hardness (9 1/4 on the Moh’s scale, as compared to 10 for diamond).
Recently, C3 Inc. along with Cree Research, another company based in Research Triangle Park, North Carolina, have developed a process to produce near-colorless synthetic moissanite that is suitable for jewelry use. It is our understanding that C3 Inc. intends to sell cut stones up to 2 carats in weight in round brilliant and other popular styles.
A Look At 23 Stones
Twenty-three faceted pieces of synthetic moissanite, weighing from 0.09 to 1.12 carats, were made available to GIA for our study. When color graded according to GIA’s D-to-Z system, the eighteen larger samples (those above 0.20 carat) ranged from near-colorless (the best being equivalent to “I”) to light yellow, light green or light gray (down to the equivalent of the “U-to-V” range). There were no eye-visible inclusions in any of the samples (on the diamond clarity grading scale, these samples would fall into the “VVS” to “SI” grades).
Synthetic moissanite has some distinctive gemological properties, as summarized on the accompanying chart (in comparison to diamond, synthetic diamond, and cubic zirconia). It can be identified based upon these gemological properties, even if one does not have access to the new test instrument developed by C3 Inc.
The most distinctive properties of the synthetic moissanites we examined are as follows:
1). The material is described as being near-colorless, but it has a more greyish, greenish, or yellowish appearance in comparison to typical near-colorless diamonds.
2). It appears to be less brilliant than diamond, but at the same time, it can show more dispersion than either diamond or cubic zirconia.
3). Diamond and cubic zirconia are isotropic optical materials (which means that they each have only one refractive index), whereas synthetic moissanite is anisotropic (it has two refractive indices, or is doubly refractive). When examining a faceted synthetic moissanite with a gemological microscope or a loupe, and by looking through the stone, the facet junctions on the reverse side of the stone will appear to be doubled. This is especially apparent when looking through the crown facets at the facet junctions on the pavilion. This optical doubling gives rise to a slightly hazy or fuzzy overall face-up appearance of synthetic moissanite in
comparison to that of diamond.
4). Because of the difference in hardness, the synthetic moissanite samples we examined were not as well polished as a typical diamond. Most of the samples displayed slightly rounded facet junctions. In addition, we noticed that the polishing lines on the facets of a synthetic moissanite were oriented in the same direction, in contrast to the situation with diamond where polishing takes place in different directions. Finally, the girdle area of most of the synthetic moissanites was frosted, polished,
or striated in appearance.
5). The material does contain whitish-appearing, needle-like inclusions that are parallel to one another, but it is free of fractures. In general, the material does not fluoresce to ultraviolet radiation.
The material will not be correctly identified if tested with a conventional thermal inertia meter. The new test instrument from C3 Inc. will distinguish synthetic moissanite from diamond. However, if one is aware of the gemological characteristics mentioned above, this new imitation can be quickly recognized with only a loupe.