Alumina is an inorganic substance with the chemical formula of Al2O3. It is a high hardness compound with a melting point of 2054℃ and a boiling point of 2980℃. It is an ionic crystal, which can be ionized at high temperature and is often used to make refractories.

Industrial alumina is prepared from bauxite (Al2O3 3H2O) and diaspore. For Al2O3 with high purity, it is generally prepared by chemical method. There are many isomorphic crystals of Al2O3, including 10, and there are mainly three crystal forms, namely α-Al2O3, β-Al2O3 and γ-Al2O3. Among them, the structure and properties are different, and it almost completely transforms into α-Al2O3 at the high temperature above 1300℃.