Corundum is an extremely hard mineral in crystalline form composed of aluminum oxide with trace amounts of titanium, iron and chromium. Naturally clear, the mineral takes on different colors when other minerals or elements are present as impurities. An extremely hard mineral (Mohs 9.0), it is used extensively in jewelry and as an abrasive.
The word corundum came into English in 1728 from the Tamil word குருந்தம், kuruntam, meaning ruby or very hard mineral, and is related to the Sanskrit word kuruvinda. Corundum has been artificially produced since 1837, and the clearest gemstones are prized as rubies, sapphires and padparascha.
Image of Montana corundum courtesy Orbital Joe, used with permission under a Creative Commons 3.0 license.