Corundum, a mineral compound formed of aluminum and oxygen. Colored varieties of corundum, such as rubies and sapphires, are prized as gems. The color is produced by trace amounts of iron, titanium, chromium, or other metals. Emery is a dark rock that is formed from a mixture of corundum and other minerals such as magnetite or spinel. Corundum ranks just below diamond in hardness. It has long been used as an abrasive, though for many applications it has been replaced by synthetic abrasives.

The world's largest deposits of corundum are in Russia, in the Urals and central Siberia. Zimbabwe, India, South Africa, and a number of other countries, including Malawi, Mozambique, and Madagascar, also have substantial deposits. Corundum has been mined at various times in Georgia, North Carolina, and Ontario, Canada. Synthetic corundum for abrasive use is made by melting bauxite in an electric furnace.

Chemical formula: Al2O3, aluminum oxide. Specific gravity: 4.02. Hardness: 9.0 (Mohs scale).