~Aluminium oxide~ (Al_{2}O_{3}). This substance occurs in several forms in nature. The relatively pure crystals are called corundum, while emery is a variety colored dark gray or black, usually with iron compounds. In transparent crystals, tinted different colors by traces of impurities, it forms such precious stones as the sapphire, oriental ruby, topaz, and amethyst. All these varieties are very hard, falling little short of the diamond in this respect. Chemically pure aluminium oxide can be made by igniting the hydroxide, when it forms an amorphous white powder: 2Al(OH)_{3} = Al_{2}O_{3} + 3H_{2}O. The natural varieties, corundum and emery, are used for cutting and grinding purposes; the purest forms, together with the artificially prepared oxide, are largely used in the preparation of aluminium. ~Aluminium hydroxide~ (Al(OH)_{3}). The hydroxide occurs in nature as the mineral hydrargyllite, and in a partially dehydrated form called bauxite. It can be prepared by adding ammonium hydroxide to any soluble aluminium salt, forming a semi-transparent precipitate which is insoluble in water but very hard to filter. It dissolves in most acids to form soluble salts, and in the strong bases to form aluminates, as indicated in the equations Al(OH)_{3} + 3HCl = AlCl_{3} + 3H_{2}O, Al(OH)_{3} + 3NaOH = Al(ONa)_{3} + 3H_{2}O. It may act, therefore, either as a weak base or as a weak acid, its action depending upon the character of the substances with which it is in contact. When heated gently the hydroxide loses part of its hydrogen and oxygen according to the equation Al(OH)_{3} = AlO.OH + H_{2}O. This substance, the formula of which is frequently written HAlO_{2}, is a more pronounced acid than is the hydroxide, and its salts are frequently formed when aluminium compounds are fused with alkalis. The magnesium salt Mg(AlO_{2})_{2} is called spinel, and many other of its salts, called aluminates, are found in nature. When heated strongly the hydroxide is changed into oxide, which will not again take up water on being moistened.