9. _Chrysolite_, called _Peridot_ by Haüy; probably the topaz of the
ancients, as our topaz was their chrysolite. It is the softest of the
precious stones, being scratched by quartz and the file. It refracts
double.
10. _Quartz_, including, as sub-species, _Amethyst_, _Rock-crystal_,
_Rose-quartz_, _Prase_ or _Chrysoprase_, and several varieties of
calcedony, as _Cat’s eye_, _Plasma_, _Chrysoprase_, _Onyx_, _Sardonyx_,
&c. Lustre vitreous, inclining sometimes to resinous; colours, very
various; fracture conchoidal; hardness, 7; specific gravity, 2·69.
11. _Opal_, or uncleavable quartz. Fracture, conchoidal; lustre,
vitreous or resinous; colours, white, yellow, red, brown, green, gray.
Lively play of light; hardness, 5·5 to 6·5; specific gravity, 2·091. It
occurs in small kidney-shaped and stalactitic shapes, and large tuberose
concretions. The phenomena of the play of colours in precious opal has
not been satisfactorily explained. It seems to be connected with the
regular structure of the mineral. Hydrophane, or oculis mundi, is a
variety of opal without transparency, but acquiring it when immersed in
water, or in any transparent fluid. Precious opal was found by Klaproth
to consist of silica, 90; water, 10; which is a very curious
combination. Hungary has been long the only locality of precious opal,
where it occurs near Caschau, along with common and semi-opal, in a kind
of porphyry. Fine varieties have, however, been lately discovered in the
Faroe islands; and most beautiful ones, sometimes quite transparent,
near Gracias a Dios, in the province of Honduras, America. The red and
yellow bright coloured varieties of fire-opal are found near Zimapan, in
Mexico. Precious opal, when fashioned for a gem, is generally cut with a
convex surface; and if large, pure, and exhibiting a bright play of
colours, is of considerable value. In modern times, fine opals of
moderate bulk have been frequently sold at the price of diamonds of
equal size; the Turks being particularly fond of them. The estimation in
which opal was held by the ancients is hardly credible. They called it
Paideros, or Child beautiful as love. Nonius, the Roman senator,
preferred banishment to parting with his favourite opal, which was
coveted by Mark Antony. Opal which appears quite red when held against
the light, is called _girasol_ by the French; a name also given to the
sapphire or corundum asterias or star-stone.
12. _Turquois_, or _Calaite_. Mineral turquois, occurs massive;
fine-grained impalpable; fracture conchoidal; colour, between a blue and
a green, soft, and rather bright; opaque; hardness, 6; spec. grav. 2·83
to 3·0. Its constituents are, alumina, 73; oxide of copper, 4·5; oxide
of iron, 4; water, 18; according to Dr. John. But by Berzelius, it
consists of phosphate of alumina and lime, silica, oxides of copper and
iron, with a little water. It has been found only in the neighbourhood
of Nichabour in the Khorassan, in Persia; and is very highly prized as
an ornamental stone in that country. There is a totally different kind
of turquois, called _bone turquois_, which seems to be phosphate of lime
coloured with oxide of copper. When the oriental stone is cut and
polished, it forms a pleasing gem of inferior value. _Malachite_, or
mountain green, a compact carbonate of copper, has been substituted
sometimes for turquois, but their shades are different. Malachite yields
a green streak, and turquois a white one.