A white or colourless oxide of aluminium occurring in two main forms. The stable form α‎-alumina (r.d. 3.97; m.p. 2015°C; b.p. 2980 ±‎ 60°C) has colourless hexagonal or rhombic crystals; γ‎-alumina (r.d. 3.5–3.9) transforms to the α‎-form on heating and is a white microcrystalline solid. The compound occurs naturally as corundum or emery in the α‎-form with a hexagonal-close-packed structure of oxide ions with aluminium ions in the octahedral interstices. The gemstones ruby and sapphire are aluminium oxide coloured by minute traces of chromium and titanium respectively. A number of other forms of aluminium oxide have been described (β-, δ‎-, and ζ‎-alumina) but these contain alkali-metal ions. There is also a short-lived spectroscopic suboxide AlO. The highly protective film of oxide formed on the surface of aluminium metal is yet another structural variation, being a defective rock-salt form (every third Al missing).

Pure aluminium oxide is obtained by dissolving the ore bauxite in sodium hydroxide solution; impurities such as iron oxides remain insoluble because they are not amphoteric. The hydrated oxide is precipitated by seeding with material from a previous batch and this is then roasted at 1150–1200°C to give pure α‎-alumina, or at 500–800°C to give γ‎-alumina. The bonding in aluminium hydroxide is not purely ionic due to polarization of the oxide ion. Although the compound might be expected to be amphoteric, α‎-alumina is weakly acidic, dissolving in alkalis to give solutions containing aluminate ions; it is resistant to acid attack. In contrast γ‎-alumina is typically amphoteric dissolving both in acids to give aluminium salts and in bases to give aluminates. α‎-alumina is one of the hardest materials known (silicon carbide and diamond are harder) and is widely used as an abrasive in both natural (corundum) and synthetic forms. Its refractory nature makes alumina brick an ideal material for furnace linings and alumina is also used in cements for high-temperature conditions. See also aluminium hydroxide.