Moissanite is naturally occurring silicon carbide and its various crystalline polymorphs. It has the chemical formula SiC and is a rare mineral, discovered by the French chemist Henri Moissan in 1893. Silicon carbide is useful for commercial and industrial applications due to its hardness, optical properties and thermal conductivity. The crystalline structure is held together with strong covalent bonding similar to diamonds,that allows moissanite to withstand high pressures up to 52.1 gigapascals. Colors vary widely and are graded from D to K range on the diamond color grading scale. Moissanite was introduced to the jewelry market in 1998. Moissanite is regarded as a diamond alternative, with some optical properties exceeding those of diamond. It is marketed as a lower price alternative to diamond that also claims less exploitative mining practices. On the Mohs scale of mineral hardness moissanite is rated as 9.5, with diamond being 10. In contrast to diamond, moissanite exhibits a thermochromism, such that heating it gradually will cause it to change color, starting at around 65 °C (150 °F). A more practical test is a measurement of electrical conductivity, which will show higher values for moissanite. Moissanite is birefringent, which can be easily seen, and diamond is not.