Aluminium oxide is an amphoteric oxide with the chemical formula Al₂O₃. It is commonly referred to as alumina (α-alumina), or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry. Its most significant use is in the production of aluminium metal, although it is also used as an abrasive owing to its hardness and as a refractory material owing to its high melting point. There is also a cubic γ-alumina with important technical applications.

Corundum is the most common naturally occurring crystalline form of aluminium oxide. Rubies and sapphires are gem-quality forms of corundum, which owe their characteristic colors to trace impurities. Rubies are given their characteristic deep red color and their laser qualities by traces of chromium. Sapphires come in different colors given by various other impurities, such as iron and titanium.

Aluminium oxide is an electrical insulator but has a relatively high thermal conductivity (30 Wm⁻¹K⁻¹) for a ceramic material. In its most commonly occurring crystalline form, called corundum or α-aluminium oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.