Aluminum oxide exists in several different crystallographic forms, of which corundum is most common. Corundum is characterized by a high specific gravity (4.0), a high melting point (about 2,050 °C, or 3,700 °F), great insolubility, and hardness.
Aluminum oxide is the major ingredient in the commercial chemicals known as aluminas. Of the pure, inorganic chemicals, aluminas are among the largest volume produced in the world today. Rubies and sapphires are crystalline, nearly pure varieties of alumina, coloured by small amounts of impurities. Synthetic rubies and sapphires are made commercially by fusing a mixture of high-purity aluminum oxide with colouring agents in an oxyhydrogen blowpipe flame. Most are cut and drilled to form tiny “jewel” bearings in watches and various precision measuring instruments.
Activated alumina is a porous form of aluminum oxide from which much of the chemically combined water has been driven off at temperatures low enough to avoid sintering. It is chemically inert to most gases, nontoxic, and will not soften, swell, or disintegrate in water. It has the ability to adsorb and hold moisture without change in form or properties, and it has high resistance to shock and abrasion. Activated alumina is used in oil, chemical, and petrochemical industries as a dehydration agent and purifier in the manufacture of gasoline, petrochemicals, natural gas, and hydrogen peroxide.
Calcined alumina is aluminum oxide that has been heated at temperatures in excess of 1,050 °C (1,900 °F) to drive off nearly all chemically combined water. In this form, alumina has great chemical purity, extreme hardness (9 on the Mohs hardness scale, on which diamond is 10), high density, and a high melting point (slightly above 2,050 °C [3,700 °F]). It possesses good thermal conductivity, heat and shock resistance, and high electrical resistivity at elevated temperatures. This combination of properties makes calcined alumina useful in abrasives, glass, porcelains, spark plugs, and electrical insulators, but the greatest quantity of calcined alumina is used to obtain aluminum.