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It is found in syenites, peridotites, serpentine as well as in some crystalline schists and crystalline limestone. So it does not occur everywhere. The rocks concerned are igneous and metamorphic which also point to their origin.
The minerals associated with corundum are spinel, tourmaline, kyanite, magnetite, chlorite and nephelite. The gem varieties of corundum are obtained very rarely and only in a few places. The countries which produce most of the world's gem corundum are Burma, Siam, Ceylon and India (Kashmir). Gem corundum has also been obtained in Afghanistan, China,, the Ural mountains and Queensland. In the U.S.A. fine rubies have been found in the crystalline rocks of N. Carolina, and sapphires and rubies along the Upper Missouri River, near Helena, Montana. Some dark blue and greenish blue corundum have also been reported from Australia. Africa has not produced any gem corundum so far. In the U.S.A. corun­dum has been mined to a small extent at a few places, a little gem corundum being found occasionally.
(i) Afghanistan.—Rubies were obtained from Afghanistan near Jagdallak with sapphires, large quantities of garnets, spinels, etc., in a belt of highly crystalline limestone.
(ii) Upper Burma.—The ruby mines of Upper Burma form the principal source of the world's supply of gem corundum which is found in a highly crystalline limestone.
(iii) Mandalay-Sagyin hills.—This area was worked at one time for sapphires, spinels, rubies, etc. Now there is not much activity" in this area.
(iv) Myitkyina-Nanyazeik.—This area also produced rubies, sapphires and spinels from the detritus afforded by the disintegration of crystalline limestone surrounded by intrusive masses of granite. Neither ruby nor spinel appear to be abundant in this area.
(v) Ceylon.—Ceylon is also renowned for its gem-stones and the gravels containing gems are known as the ' Illam' which underlies the soil in paddy-fields.
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