Aluminium oxide is a chemical compound of aluminium and oxygen with the chemical formula ₂O₃. It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium(III) oxide. It is commonly called alumina, and may also be called aloxide, aloxite, or alundum depending on particular forms or applications. It commonly occurs in its crystalline polymorphic phase a-Al₂O₃, in which it composes the mineral corundum, varieties of which form the precious gemstones ruby and sapphire. Al₂O₃ is significant in its use to produce aluminium metal, as an abrasive owing to its hardness, and as a refractory material owing to its high melting point.