Moissanite is the name given to naturally occurring silicon carbide and to its various crystalline polymorphs. It has the chemical formula SiC and is a rare mineral, discovered by the French chemist Henri Moissan in 1893 while examining rock samples from a meteor crater located in Canyon Diablo, Arizona. At first, he mistakenly identified the crystals as diamonds, but in 1904 he identified the crystals as silicon carbide. Artificial silicon carbide had been synthesized in the lab by Edward G. Acheson just two years before Moissan's discovery.
Moissanite brings a number of really excellent things to the table if you're considering purchasing a gemstone. It is extremely tough, a 9.25 on the Moh's scale, making it slightly harder than sapphire, and very durable for every day wear. It has a higher refractive index and more dispersion than diamond, meaning this stone does *incredible* things with light. Diamond is valued (in part) for the rainbow "fire" it gives off, but moissanite actually puts off even more fire than diamond. Since it's lab created, there are no "conflict" issues surrounding it, so you can sleep at night also.