About Moissanite: Information
The synthetic form of the silicon carbide moissanite, SiC, has been manufactured for ornamental and gem use: in the 1960s some iridescent, though opaque, crystal groups were around at gem and mineral shows but it was only in recent years that a transparent variety was able to be synthesized, the manufacturer being located in North Carolina, USA. Most of the properties of diamond are quite well imitated and the usual anxiety associated with new diamond imitations was reported to be pervading the trade. Such reports are usually exaggerated – the properties of moissanite may be detected easily by a gemmologist with simple equipment.
Synthetic moissanite belongs to the hexagonal crystal system and shows birefringence absent from diamond though doubling of back facet edges cannot usually be seen through the table facet, which would have been easier
to detect, but in a direction at right angles to this. Near-parallel needles and stringers may be seen at right angles to the table. Some specimens show rounded facet edges (those of diamond are exceptionally sharp) which are not in themselves azvital clue. There are also some uni-directional polishing lines on adjacent facets, which do not occur on diamond.
Gemmological properties are: hardness 9.25, RI 2.648–2.691 with a birefringence of 0.043, uniaxial positive and dispersion 0.104, which is more than twice as great as diamond. The SG is 3.22 (diamond is 3.52). These properties can all be tested with a little effort but with any diamond imitation it is usually worth devising a catch-all detector. Reflectivity meters have usually been used to separate diamond-like transparent stones from their more serious imitators YAG and CZ though they can only reach a few stones in a piece of jewellery which contains many small ones in hard-to-reach places. These lurking ‘diamonds’ can more often be reached with the thermal conductivity tester which will very effectively separate diamond from most of its simulants.
It has been suggested that manufacturers of synthetic moissanite might be able to alter the RI of their product so that the reflectivity meters might give a ‘diamond’ reading. [Reflectivity meters do not measure RI as such but RI does influence reflectivity.] In such a case it might have been possible to repolish the specimen so that the original RI could be assessed. Rumours of this kind often circulate through the trade and those involved should ask themselves ‘what might the manufacturer gain from all this trouble?’
A synthetic near-colourless moissanite has been heat-treated, the treatment causing a brownish colour to develop across all the facets. Cleaning and hand-polishing the samples, using cerium oxide on leather, restored the reflectivity to 98% of the non-treated material. If heating forms part of any testing experiment on a suspected moissanite, the gemmologist should remember that surface oxidation could occur and keep the level of heating
to a minimum. The colour of the surface might undergo alteration.
The thermal conductivity tester will give a ‘diamond’ reading for synthetic moissanite in any case so that its use may be confined to separating diamond and synthetic moissanite from other diamond simulants: a further
test to separate the two could well be magnification. Diamond will sink and moissanite float in di-iodomethane (SG 3.34). Some coloured moissanites have been brown, green, yellow and blue but the colours are not very strong. Green specimens that I have seen are not like green diamond. A brown moissanite grown in Russia is reported to have been grown by CVD: the specimen described was opaque.
The first firm to produce synthetic moissanite and sell it (exclusively, at first) was C3 Inc. in North Carolina, USA. The same firm, now called Charles & Colvard, produced a tester, which they sold under the name Tester Model 590. Other instruments have followed. These separate moissanite from diamond by scanning the blue and near- visible UV areas of their absorption spectrum. In moissanite there is an intense region of absorption extending down from about 425 nm to the UV region. Colourless diamond, on the other hand, transmits well down into the UV.
A halogen light source directs a beam on to the table facet which reflects it. If it transmits wavelengths from the blue to the UV region the tester gives a visual indication plus a bleep, indicating that the specimen is diamond.
If no response is given the specimen will be moissanite, having absorbed this range of wavelengths.
Another instrument, the Presidium moissanite tester, detects the very small current passed by semiconducting materials. The operator receives a signal indicating diamond/not diamond. As most diamonds, apart from type IIb blue diamonds, are not semi-conductors, any current detected may be caused by impurities in the material.
In a paper in Australian Gemmologist 20, 483–85, 2000, the authors found that the testing of synthetic moissanite was made easier by the Presidium tester. As synthetic moissanite is a semiconductor, the instrument is able to sense a forward leakage of current. Other testers are based on the ‘breakdown voltage’ (which overcomes resistance to an insulator causing current to flow). The authors found that all synthetic moissanite specimens were
detected as such by the apparatus, indicating them by the illumination of a bright red window display and a sound alert. Other species caused a green ‘Test’ lamp to be illuminated. False positive synthetic moissanite readings occurred during the evaluation, particularly when the tip of the probe was in contact with metal (such as the setting) and also when a germanium transistor was touched. A synthetic moissanite response was also given by a black electrically conductive industrial-quality diamond.