In
nature spinel is a constant companion of corundum. It occurs in the
same rocks under the same conditions but has some magnesia in addition.
Good spinels have been obtained from Ceylon, Siam and Burma. It is also
found in Mysore, Madagascar, Australia, Afghanistan and Brazil. The
spinel occurs in the crystalline limestone with corundum or ruby in
the Mogok stone tract, and has been found under the same conditions as
the ruby. In the presence of magnesia, the 
forms spinel instead of corundum.
It
is also found in serpentine, gneiss and in gravels in Ceylon, Burma and
Siam. Spinel has also been reported from the old ruby mines of
Afghanistan and the sands of the Irrawaddy above Myitkyina. Spinels
occur in good crystals more often than the ruby, but larger crystals
are rare. Crystals of good ruby colour only fetch about half the price
of the ruby. In Ceylon Balas ruby, blue spinel and green spinel are
found. In gem sands, three-fourths of the material is spinel. The
spinel may also be opaque, dull and semi-transparent. Other varieties
of spinel occur in the following places: blue spinel—Baluchistan;
chloro-spinels—Aker, Sweden, Antwerp and Ceylon; black spinel
(Ceylonite)—Ceylon, Bohemia, Tyrol, Russia, etc.; Automolite (zinc
spinel)—Sweden; Balas from Ballahia on the Upper Oxus described by
Marco Polo.
Industry
The
spinel is usually obtained along with the ruby and sapphire, and is
sold along with cheap rubies and sapphires by vendors at Mogok.
Nowadays synthetic spinels of gem quality are made with a wide range of
colours in large amounts by the Verneuil process.
The spinel is cut on an iron wheel with emery or diamond powder and polished on a copper one. The form is usually
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