Aluminum oxide mineral (AlO) that is, after diamond, the hardest known natural substance. Gem varieties are sapphire and ruby; mixtures with iron oxides and other minerals are called emery. Corundum is widespread in nature, although large deposits are rare. Rich deposits occur in India, Russia, Zimbabwe, and South Africa. In addition to being a precious gem, corundum is used as an abrasive for grinding optical glass and for polishing metals and has also been made into sandpapers and grinding wheels. For most industrial applications, however, it has been replaced by synthetic materials such as alumina; synthetic corundum is also manufactured.
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