Silicon carbide (SiC) is a ceramic compound of silicon and carbon.
The word moissanite is a trade name given to silicon carbide for use in the gem business.
Most silicon carbide is man-made for use as an abrasive (when it is often known by the trademark carborundum), or more recently as a semiconductor and moissanite gemstones. The simplest manufacturing process is to combine silica sand and carbon at a high temperature, between 1600°C and 2500°C.
The material formed in the Acheson furnace varies in purity, according to its distance from the graphite resistor that is the heat source. Clear, pale yellow and green crystals have the highest purity, and are found closest to the resistor. The colour changes to blue and black at greater distance from the resistor, and these darker crystals are less pure, and usually doped with aluminium, which decreases electrical conductivity.
Purer silicon carbide can be made by the more expensive process of chemica vapor deposition. Commercial large single crystal silicon carbide is grown using a physical vapor transport commonly known as modified Lely method.

Discovery
The material was discovered by Edward Goodrich Acheson in 1893 or 1893, and he not only developed the electric batch furnace by which SiC is still made today, but also formed The Carborundum Company to manufacture it in bulk, initially for use as an abrasive. It is said that Acheson was trying to dissolve carbon in molten corundum (alumina) and discovered the presence of hard, blue-black crystals which he believed to be a compound of carbon and corundum: hence carborundum. Or, he named the material carborundum by analogy to corundum, which is another very hard substance (9 on the Mohs scale).

Properties
Alpha silicon carbide (a-SiC) is most common, and is formed at temperatures >2000°C. Alpha SiC has the typical hexagonaln crystal structure. Beta modification (ß-SiC), with a face-centered cubic crystal structure, is formed at temperatures of below 2000°C, but has relatively few commercial uses. Silicon carbide has a specific gravity of 3.2, and its high melting point (approximately 2700 °C) makes silicon carbide useful for bearings and furnace parts. It is also highly inert chemically. There is currently much interest in its use as a semiconductor material in electronics, where its high thermal conductivity, high electric field breakdown strength and high maximum current density make it more promising than silicon for high-powered devices. In addition, it has strong coupling to microwave radiation and that, together with its high melting point permits practical use in heating and casting metals. SiC also has very low thermal expansion coefficient and no phase transitions that would cause discontinuities in thermal expansion.
Pure SiC is clear. The brown to black color of industrial product is caused by iron impurities. The rainbowish lustre of the crystals is caused by the passivation layer of silicon dioxide that forms on its surface.
As a gemstone, silicon carbide is similar to diamond in several important ways: it is transparent and extremely hard (9.25 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.

Found in nature
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. Moissan's discovery of naturally occurring SiC was disputed at first because his sample may have been contaminated by silicon carbide saw blades that were already on the market at that time.

Uses
Semiconductor
Structural material
Astronomy

Silicon carbide's hardness and rigidity make it a desirable mirror material for astronomical work, although they also make manufacturing and figuring such mirrors quite difficult.
Silicon carbide may be a major component of the mantles of as-yet hypothetical carbon planets.

Grit
Disc brake
Diesel Particulate Filter
Cutting Tools
Heating Element
As a gemstone

In 1998 C3, Inc. (Charles and Colvard) [Nasdaq: CTHR], 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. For example, a 1 carat moissanite gem sells for about $600 (2005 USD), while a diamond of similar size and color typically runs for upwards of $4500. Synthetic moissonite is approximately as hard as diamond, with a slightly higher index of refraction and greater dispersion; these qualities make SiC a decent and durable diamond simulant. It is interesting to note that the higher dispersion and index of refraction means moissanite has more fire and brilliance than diamond.
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 harder than cubic zirconia (9 1/4 vs. 8 1/2), lighter (SG 3.33 vs. 5.6), and much more resistant to heat. This results in a stone of higher lustre, sharper facets and good resilience: loose moissanites may be placed directly into ring moulds, the stones remain undamaged from temperatures up to twice the 900°C melting point of 18k gold.

In popular culture
In 2001: A Space Odyssey and the related series of books and movies (by Arthur C. Clarke and Stanley Kubrick, among others) the monoliths (or at least their exteriors) were made of silicon carbide.