Al2O3

Aluminum oxide, with the chemical formula Al2O3, also known as alumina, is a compound of aluminum and oxygen. In nature it occurs as corundum, a mineral that forms colorless crystals but hosts trace elements that give sapphire (blue, among others) and ruby (red). Hydrated forms such as Al2O3·xH2O occur in bauxite, the primary ore for aluminum.

Aluminum oxide is an extremely hard, chemically inert solid. The most stable crystalline phase is alpha-alumina, also called corundum, which has a Mohs hardness of 9 and a high melting point around 2072°C. It is a good electrical insulator and exhibits high dielectric strength. It resists attack by most acids and bases at room temperature, though it can be attacked by molten alkalis and hydrofluoric acid. It has a relatively low density of about 3.95 g/cm3 and is optically transparent when fabricated as a high-purity single crystal, as in sapphire and ruby.

Alumina is typically produced from bauxite by the Bayer process, then refined to pure Al2O3. It can be melted and refined to produce metallic aluminum via electrolysis (Hall-Héroult process). In industry, alumina is used as an abrasive, a refractory material, and a support or coating in catalysts. It is also used to make advanced ceramics, spark plug insulators, and as a substrate for LEDs and protective coatings.

As a versatile material, alumina’s combination of hardness, thermal stability, and chemical inertness underpins its broad use in engineering, electronics, and jewelry.