The chemical formula of aluminum oxide is Al₂O₃, and it is the main component of bauxite. It appears as a white powder with various crystal forms, the most common being α-Al₂O₃ and γ-Al₂O₃. α-Al₂O₃ has a hexagonal close-packed crystal structure, commonly known as corundum, with a melting point of 2045°C, a density of 3.965 g/cm³, a Mohs hardness of 8.8, and is insoluble in water, acids, or bases. γ-Al₂O₃ has a cubic close-packed crystal structure, is insoluble in water, but soluble in acids and bases, and is a typical amphoteric oxide.