The turquois is never found in crystals, but in reniform or stalactitic
masses. The color varies from pea- and apple-green to greenish-blue,
sky-blue and dark-blue.
The hardness of the turquois is 6., specific gravity 2.6 to 2.8, lustre
waxy, and condition opaque to slightly translucent.
Before the reducing flame of the blow-pipe, the turquois does not melt,
but becomes brown and colors the flame green. With borax and salts
of phosphorus the turquois melts to a clear glass, while it is also
soluble in hydrochloric acid. Oriental or mineral turquois is composed
of:
Alumina 47.45
Phosphoric acid 27.34
Water 18.18
Oxide of copper 2.02
Iron 1.10
Oxide of manganese 0.50
Phosphate of lime 3.41
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The best color is a clear deep sky-blue, and in the true turquois this
color improves by artificial light; imitation turquoises, however, lose
their fine color under the same conditions.
The finest gem turquoises come from the northeastern part of Persia,
between Nishapoor and Meshed. Here they are mined and partly cut, and
then the Persian merchants carry them to Russia, where they are sold
at the great annual fair of Nijni-Novgorod and in Moscow. Mineral
turquoises are also found in New Mexico, Arizona, and Nevada, but not
of sufficient size or sufficiently good color to make gem stones,
although they are prized for collections. Specimens are also found in
Burmah, Khorassan, Thibet, China, Silesia, Saxony, and on the Isthmus
of Suez. The stones from these places have, as a rule, but little
value, as the color fades or turns green from exposure to the light.
Of late however, some very good turquoises have come from Egypt. The
color of a faded Persian turquois can sometimes be restored by simply
repolishing the stone.
Occidental or bone turquoises called new rock or odontolites, to
distinguish them from the Persian or old rock stones, are of organic
origin.
They are cut from the teeth of mammoths, mastodons, dinotheriums, etc.,
and are found near the town of Simor, in Lower Languedoc, France.
These teeth, the enamel of which is nearly as hard as the mineral
turquois, are colored by contact with phosphate of iron and copper,
which gives them a dark-blue, light-blue, and bluish-green color. They
are easily attacked by a file, and totally destroyed by aqua-fortis.
When heated, the fossil turquois or odontolite gives an offensive odor,
owing to the decomposition of animal matter.
The odontolite is lighter than the mineral turquois, changes color by
artificial light, loses color in distilled water and alcohol, and is
translucent on the edges.
This fossil turquois does not fade like the mineral turquois, but by
artificial light appears of a dirty grayish-blue.
Turquoises are sometimes artificially stained, but this can be detected
by applying a drop of ammonia to the back of the stone, and if the
color is artificial the ammonia will eat it off, leaving a green spot.
Ammonia does not affect the color of the Persian turquois. The so-called
“reconstructed” turquoises are very close imitations of the real, but
are easily distinguished, as they change rapidly to a deeper blue when
immersed in water, and while wet the surface of the stone shows cracks
in every direction. These stones become softer through soaking in water
or alcohol. The original color, however, returns when the stone is dry,
but the cracks remain in faint outline.