Corundum

Corundum: A compound of elements Corundum is a mineral consisting of aluminum oxide with a chemical formula Al2O3. Aluminum is the most abundant metallic material in the crust of the earth; only the oxygen and silicon are found in greater abbundance. Aluminum is never found as a free metal. It is commonly found as aluminum silicate or as a silicate of aluminum mixed with other metals such as sodium, potassium, iron, calcium, and magnesium. …

…the aluminum oxide (Al2O3) of which the mineral is composed. Excellent sapphires occur in alluvial deposits of sand and gravel in Sri Lanka, Thailand, India, Afghanistan, China, the Ural Mountains and elsewhere in the former Soviet Union, and Queensland, Australia. In the United States sapphires are found in the corundum deposits of Macon County, North Carolina, and near Helena, Montana. Sapphires are used, as are rubies, as hard bearings in watches and scientific apparatus.