CORUNDUM, a mineral of the composition able for its hardness and forming in its finer varieties a valuable gem-stone (Germ. Korund, Fr. Corindon). The name is believed to be derived from Kurund (Hindi) or Kurundam (Tamil), the native name of the stone in India whence specimens were first sent to England. The transparent coloured varieties are known as ruby and sapphire, whilst the impure massive forms are known as emery (q.v.). Corundum crystallizes in the rhombohedral system and shows some variety of habit, the commonest being acute hexagonal bipyramids, sometimes in barrel-shaped forms, or tabular owing to the predominance of the basal plane. Corun dum has no true cleavage but a parting parallel to the base and the rhombohedron, the latter a plane of lamellar twinning, some times secondarily developed due to pressure.
Next to diamond, corundum is the hardest known mineral (9 on Mohs' scale), this property being sufficient to distinguish it from all other minerals. The pure mineral is colourless, small quantities of impurities (iron, chromium, titanium) being responsible for the wide range of colours observed in natural crystals. Alteration products such as gibbsite, diaspore, margarite and bauxite often accompany corundum. It is used largely for watch jewels, bear ings in electrical apparatus and, as emery, extensively as a pol ishing substance. Corundums fit for gem-stones come principally from Ceylon, Burma, Siam and Montana ; they are largely re covered from gravels and residual surface deposits. Crystalline is dimorphous, a second hexagonal form occurring as a phase in investigations on the system Mg0—Al203. #Al203 is not convertible into corundum (aAl203).
Alumina melts have a great power of crystallization, rendering it difficult to preserve glasses of this composition. The melting point of the artificial corundum is 2,o5o° C. As substitutes for natural gems, synthetic rubies and sapphires are manufactured, many of which are distinguishable only by the expert.
Corundum is of widespread distribution and occurs both in igneous and metamorphic rocks.