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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