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SPINEL
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(RUBY SPINEL, BALAS RUBY)
The
group of spinel includes in mineralogy a number of species of different
though analogous composition. The spinel employed as a gem is almost
wholly a magnesium aluminate, having the percentage composition:
alumina 71.8 and magnesia 28.2. This is usually of a red color,
different shades giving gems known by different names as follows : Deep
red, spinel-ruby; rose-red, balas ruby; yellow or orange red,
rubicelle; violet-red, almandine ruby. Spinel is thus known among gems
chiefly -as a relative of the ruby, and this sort of spinel will first
be considered.
The
spinel rubies differ from the true or corundum rubies in hardness,
specific gravity, and system of crystallization. The hardness of spinel
is 8, or about that of topaz, and the specific gravity 3.6. It is thus
neither as hard nor as heavy as corundum ruby. Again, the system of
crystallization differs. Spinel crystallizes in the isometric system,
and is usually found in the form of octahedrons, while corundum ruby is
hexagonal in crystallization. (See colored plate.) Spinel is singly
refracting in polarized light, and corundum doubly refracting. Spinel
ruby is infusible before the blowpipe, but on heating undergoes a
curious series of changes in color which are quite characteristic. The
red changes first to brown, and then becomes black and opaque, but on
cooling the black changes to green, then becomes nearly colorless, and
finally the stone resumes its original red color. As a small percentage
of chromium is usually found by analysis to exist in ruby spinel, its
color is generally considered to be due to this ingredient. While the
spinel ruby is considered of less value than the corundum ruby, and is
sometimes by fraud or error substituted for the latter, it yet has a
definite value as a gem when sold under the name of spinel ruby or some
of its varieties. This value is usually reckoned at about half that of
the corundum ruby, although variations in quality of the stones, as
well as changes in demand, cause differences of price. Thus Emanuel
mentions a spinel ruby of good quality weighing 40 carats, which in
1856 was sold for $2,000, but in 1862 brought at public auction only
$400. In 1866, however, it was again sold for $1,200. A spinel ruby
among the French crown jewels,
95
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