Korundum

Korundum, also known by the mineral name corundum, is a mineral consisting of aluminum oxide (Al2O3). It crystallizes in the hexagonal system and typically occurs as hexagonal prismatic crystals or tabular crystals. With a hardness of 9 on the Mohs scale and a density around 3.95 to 4.05 g/cm3, corundum is one of the hardest natural minerals. Pure, colorless corundum is transparent; most specimens acquire color from trace impurities rather than structural defects.

Corundum forms in a variety of settings, typically in aluminum-rich rocks under high-temperature and high-pressure conditions. It occurs in metamorphic and igneous environments and can also weather to detrital grains that are transported and deposited in sedimentary deposits.

Color in corundum arises from trace elements. Red varieties are rubies (chromium), blue varieties are sapphires (iron and titanium), and other colors produce pink, orange, yellow, green, or purple sapphires or rubies. A rare colorless form exists as a gemstone. Star corundum and other color phenomena result from inclusions or diffusion of other elements, such as rutile needles that form asterisms.

Uses and production: Corundum has long served as an industrial abrasive, notably as emery for grinding and cutting. Synthetic corundum is widely produced by methods such as the Verneuil process and flame fusion; synthetic sapphire is used for watch crystals, optics, and laboratory equipment, while synthetic ruby is used as a gemstone and in industrial applications.

Name and classification: The mineral name corundum (korundum in some languages) refers to the same Al2O3 component. Gem-quality corundum comprises the red variety known as ruby and all non-red varieties collectively called sapphire.