TURQUOISE, ODONTOLITE, VARISCITE
Of all the opaque stones turquoise (Plate XXIX, Fig. 17) alone finds a
prominent place in jewellery and can aspire to rank with the precious
stones. The colour varies from a sky-blue or a greenish blue to a
yellowish green or apple-green. Only the former tints, which are at
the same time the rarer, are in general demand, and they possess the
great advantage of harmonizing with the tint of the gold setting.
The blue colours are, especially in the case of the Siberian stones,
by no means permanent, and fade in course of time. Turquoise is
amorphous and seldom crystalline, and is therefore somewhat porous; it
should consequently never be immersed in liquids or be contaminated
with greasy and dirty matter lest the dreaded change of colour be
brought about. The stones are translucent in thin sections, and a
good observation is possible with the refractometer if the back of
the stone is flat and polished, since only the section immediately
adjacent to the instrument is concerned; the refractive index is about
1·61. The specific gravity varies from 2·75 to 2·89. Turquoise has a
hardness of slightly under 6 on Mohs’s scale, and takes a good polish,
which is fairly durable, since on account of the comparative opacity
of the stones scratches on the surface are not very noticeable.
In composition it is a complex phosphate of aluminium and copper,
corresponding to the formula CuOH.[6Al(OH)_{2}].H{5}.(PO_{4})_{4},
with ferric oxide replacing some alumina. The blue colour is due to
the copper constituent, and the predominance of iron may cause the
greenish shades; but the water contained in the stones plays no mean
part, since they turn a dirty green when it is driven off. The faded
colour can sometimes be restored by immersion of the stone in ammonia
and subsequent application of grease, but the effect is not lasting.
Attempts are sometimes made to improve inferior stones by impregnating
them with Berlin blue, but with only qualified success. Turquoises are
said to be affected by the perspiration from the skin.
The name of the species comes from a French word meaning Turkish, and
arises from the fact that the gem-stone first reached Europe by way of
Turkey. Another, but less obvious, suggestion is that it is derived
from the Persian name for the species, _piruzeh_. Our turquoise and
other phosphates of similar appearance were probably known to Pliny
under the three names _callais_, _callaina_, and _callaica_.
The finest turquoise still comes from the famous mines near Nishapur in
the Persian province of Khorassan, where it was known in very ancient
times; it is found with limonite filling the cracks and cavities
in a brecciated porphyritic trachyte. Pieces of the turquoise and
limonite from here are sometimes cut without removal of the latter,
and sold as ‘turquoise-matrix,’ when the precious stones are too tiny
to be worth separate working. It also occurs at Serbâl in the Sinai
Peninsula. Among the more recent localities may be mentioned Los
Cerillos Mountains, New Mexico; Sierra Nevada, Nevada, where pale blue
and green stones are found; San Bernardino County, California, where
again the stones are rather pale; and Arizona, where it occurs in pale
greenish-blue stones.
Some of the stones that have been seen are not the true turquoise but
odontolite, or bone turquoise, which consists of the teeth and bones
of mastodon or other extinct animals, phosphate of iron being the
colouring material. These stones may easily be recognized by their
organic structure, which is clearly visible if viewed with a strong
lens or under the microscope. Moreover, odontolite invariably contains
some calcium carbonate, and effervescence takes place if it be touched
with hydrochloric acid. Turquoise dissolves in hydrochloric acid, but
without effervescence, and since it contains copper, a fine blue colour
is imparted to the solution by the addition of ammonia. Odontolite has
a higher specific gravity, 3·0 to 3·5, but lower hardness, 5 on Mohs’s
scale.
Variscite, the hydrated phosphate of aluminium, corresponding to the
formula AlPO_{4} + 2H_{2}O, is found in masses resembling a greenish
turquoise, but it is much softer, being only 4 on Mohs’s scale. The
specific gravity is 2·55. Round nodular masses of variscite are found
in Utah.