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TURQUOIS
This
mineral differs from nearly all others held in favor as gems in not
being transparent, and never- occurring in the form of well-defined
crystals. In composition turquois is a hydrous phosphate of aluminum,
the percentages being: water, 20.6 per cent, alumina, 46.8 per cent,
and phosphorus oxide, 32.6 per cent. Thus, in composition as well as
opacity, turquois differs from most other gems, they being usually
silicates, or some form of silica. Besides the above ingredients
turquois always contains a small percentage of copper oxide, and
usually iron, calcium, and manganese oxides in small amount. It is the
copper compound which undoubtedly gives turquois its inimitable color,
that color to which it owes its chief charm as a gem. This color varies
from sky-blue through bluish green, and apple-green to greenish gray.
Of
these colors, the pure sky-blue, or robin's-egg blue, is by far the
most highly prized, and is, in fact, the only standard color for the
gem. Green is, however, the most common and the most lasting color of
the mineral, and it is one of the faults of the gem that the blue
shades often fade to green after being exposed to the light for a time.
In a stone of first quality, however, especially a Persian turquois,
such fading of color is exceptional. The hardness of turquois is 6. It
is, therefore, somewhat more easily scratched than other gems. Its
specific gravity varies from 2.6 to 2.8, being about that of quartz. It
does not fuse before the blowpipe; but turns brown and assumes a glossy
appearance. By the copper of the turquois the blowpipe flame is
usually colored green. When heated in a closed glass tube the mineral
turns brown, or black, and gives off water. Almost any of these tests
will serve to distinguish true turquois from stones used to imitate it.
It has a conchoidal fracture and waxy luster. On account of its opacity
it is almost never cut with facets, but in a round, or oval form, with
convex surface. The pieces desirable for cutting rarely reach a large
size, so that big gems of turquois are comparatively unknown.
Much
of the so-called turquois used in former times was bone-turquois, or
odontolite, made from fossil bone, colored by a phosphate of iron. It
is still obtained mostly from the vicinity of the town of Simor, Lower
Languedoc, France. It is sometimes known as Western, or Occidental
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