Alumina, or Oxide of Aluminium. Many precious stones, as sapphire, ruby, amethyst, etc., consist of practically pure alumina in a crystalline state. Crystalline alumina is, next to the diamond, the hardest of all known substances. Alumina, as prepared by a process of precipitation, forms a white, amorphous powder, which has a great affinity for colouring matters, combining with them to form lakes (q.v.). It is hence of great importance in dyeing and colour-manufacture.