ALUMINA, the only aluminium (q.v.) oxide. It occurs as corundum, which is a hard colourless crystalline solid, as ruby, sapphire, topaz, etc. (tinted by other metallic oxides), and in bauxite (q.v.) . Amorphous alumina may be obtained by ignition of aluminium hydrate, or aluminium sulphate, etc. Synthetic rubies are made by heating together alumina, barium fluoride and a trace of potassium dichromate. (See GEMS, ARTIFICIAL.) Alumina, when precipitated from a solution containing a colouring matter, will form an insoluble pigment, called a lake. It is also commonly used as a mordant in dyeing. Alumina is next in hard ness to the diamond (see ABRASIVES), crystallizes in the rhombo hedral system and has the chemical formula