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weighing 56 carats, was in 1791 valued at $10,000. This is much more than such a stone would now probably be worth.
Not
only is spinel ruby related to corundum ruby in color and use, but the
two are frequently associated together in nature. The gem gravels of
Ceylon, Siam, Australia, and Brazil contain both kinds of rubies, and
the ruby mines of Upper Burmah, where the corundum ruby occurs in a
crystalline limestone, produce also large quantities of spinel rubies.
Spinel rubies also come in large quantity from Badakschan, in
Afghanistan, near the river Oxus, the name of balas rubies, by which
they are often known, being said to be derived from Beloochistan,
another form of which word is Balakschan. The Persians have a
tradition regarding these mines, that they were disclosed by an
earthquake which rent the mountain in twain.
The
localities above mentioned furnish nearly all the spinel rubies of
commerce. A few have been found in North America, Hamburgh, New Jersey,
and San Luis Obispo, California, being two localities where small
crystals have been obtained, but they have never afforded any
appreciable supply. No spinel rubies of great size are known. Bauer
mentions as the largest known, two cut stones, one of 81 carats, and
the other 72\ carats, exhibited at the London Exposition of
1862. The King of Oude is said at one time to have possessed a spinel
ruby the size of a pigeon's egg. Another celebrated spinel ruby is that
known as the **' Ruby of the Black Prince," which is shown among the
English crown jewels in the Tower of London.
Spinel
occurs in many other colors besides red, such as orange, green, blue,
and indigo, as well as white and black. Occasionally colorless spinels
occur, and as they cannot be distinguished by their behavior in
polarized light from the diamond, it is sometimes sought to substitute
them for the latter. They can be detected at once, however, by their
inferior hardness. While spinels of any color, if transparent and free
from flaws, make desirable gems, the only colors found in sufficient
quantity outside of the red to make an appreciable supply are the blue
and the black. The blue spinels resemble the sapphire in color, though
they are somewhat paler. They come chiefly from Ceylon and Burmah,
where they occur together with the ruby spinel. The black spinel is
known as ceylonite, or pleonaste, and is also obtained chiefly from
Ceylon, although occurring of a quality suitable for cutting at Mount
Vesuvius in Italy.
Like
the ruby, spinel can be made artificially, the process being to heat a
mixture of alumina and magnesia with boracic acid, and if a red color
is desired, a little chromium oxide. No attempt seems to have been made
as yet, however, to manufacture it for gem purposes.
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