Moissanite is a trade name name given to silicon carbide (chemical formula SiC) for use in the gem business. As a gemstone, silicon carbide is similar to diamond in several important ways: it is clear and extremely hard (9.5 on the Mohs scale, compared to 10 for diamond), with an index of refraction between 2.65 and 2.69 (compared to 2.42 for diamond). SiC has a hexagonal crystalline structure.
Naturally occurring moissanite is extremely rare, as it is not formed naturally in any quantity within the Earth, and thus is found only in tiny quantities in certain types of meteorite and as microscopic traces in corundum deposits and kimberlite. Virtually all of the silicon carbide sold in the world, including moissanite gemstones, is synthetic. Natural moissanite was first found in 1905 as a small component of a meteorite in Arizona by Dr. Ferdinand Henri Moissan, after whom the material is named in the gem market. Synthetic silicon carbide has been known since 1892, when it was first produced by Eugene G. Acheson in his newly invented resistance furnace. Acheson named the material carborundum by analogy to corundum, another very hard substance (9 on the Mohs scale). Moissan's discovery of naturally occurring SiC was disputed at first due to the potential for contamination of his sample by silicon carbide saw blades that were already on the market at that time.
In 1998 C3, Inc. (Charles and Colvard), a subsidiary of Cree Research, Inc., introduced gem-quality synthetic silicon carbide onto the market under the name "moissanite," marketing it as a lower-cost alternative to diamond. Touted as the best, most convincing diamond simulant to date, moissanite kicked up quite a stir: Journalists and programmes such as Nova reported on the troubling ease of misidentification between moissanite and diamond, citing incidents of fraud.
While some properties of moissanite are closer to diamond than those of cubic zirconia, another synthetic diamond simulant, once its properties are known, moissanite is perhaps even easier to identify. Jewellers were at first fooled by moissanite's thermal conductivity which approximates that of diamond, rendering older thermal testers useless; what worked with cubic zirconia did not work with moissanite.
Moissanite is much harder than cubic zirconia (9.5 vs. 8.5), lighter (SG 3.33 vs. 5.6), and much more resistant to heat. This results in a stone of higher lustre, sharper facets and extraordinary resilience: loose moissanites may be placed directly into ring moulds, the stones surviving unscathed from temperatures up to twice the 900°C melting point of 18k gold.
Unlike isometric diamond and cubic zirconia, the polytype of moissanite presently synthesized is hexagonal: being doubly refractive, the stones are easily detected with a polariscope or even by eye, the strong birefringence being seen as a doubling of the stone's back facets. Moissanite is usually cut with its optic axis perpendicular to the table of the stone in order to minimize this "drunk vision" effect, but even a slight tilt will betray the stone's true nature.
Moissanite has a dispersive power nearly 2.5 times greater than diamond (0.104 vs. 0.044); this means the 'fire' of moissanite is viewed as excessive by some (yet more beautiful by others).
To date all moissanites have been plagued by a muddy tinge, usually green or gray. Lacking the brilliant whiteness of cubic zirconia, most moissanites would grade within the I-K colour range of diamond. However, it should be noted at least two fancy varieties of moissanite have been produced: a blue not unlike natural blue diamonds, and a rich green somewhat resembling tsavorite.
Due to its relatively expensive manufacturing process, and because C3, Inc. is its only supplier as of 2005, moissanite is much more expensive than cubic zirconia (though it remains substantially cheaper than diamond).