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. These silicates are not useful ores and it is chemically difficult, and therefore an expensive process, to get aluminum from them. Bauxite, an impure hydrated aluminum oxide, is the commercial source of aluminum and its compounds. Oxygen makes up 21 percent by volume or 23.15 percent by weight of the atmosphere; 85.8 percent by weight of the oceans (88.8 percent of pure water is oxygen); and, as a material of most rocks and minerals, 46.7 percent by weight of the solid crust of the earth. Oxygen accounts for 60 percent of the human body. It is a important for all living tissues; almost all plants and animals, including all humans, require oxygen, in the free or combined state, to maintain life. Alumina or Aluminum Oxide Al2O3 , is an oxide found in nature as the mineral corundum, Al2O3; diaspore, Al2O3+H 2O; gibbsite, Al2O3+3H2O; and most commonly, bauxite, an impure form of gibbsite. It is the only oxide formed by the metal aluminum. The precious stones ruby and sapphire are composed of corundum colored by small amounts of impurities.
Next to the diamond, corundum is the hardest natural substance, with a relative hardness of 9 and a specific gravity of about 4. Corundum s crystal form is a hexagonal system. Gem-quality transparent crystals of the mineral form sapphires and rubies. Common corundum, which occurs either in a crystallized or granular form, is usually gray, grayish-blue, or brown; the black granular variety, which contains varying amounts of hematite and magnetite, is called emery. Common corundum and emery are both used as abrasives. Synthetic corundum, sold under various trade names, is also used as an abrasive. Synthetic rubies and synthetic sapphires have also been made. Sapphire is a precious gemstone that occurs as a transparent, blue variety of the mineral corundum. The ruby is a red variety of gem-quality corundum. Colorless, precious corundum is called white sapphire; yellow corundum is called yellow or golden sapphire, or Oriental topaz; and pale pink stones are called pink sapphire. The various colors in corundum are caused by small amounts of impurities, such as chromium, iron, or titanium, in 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.