jewellers
value it as an inferior quality of the sapphire, without paying any
regard to its phenomena of light. Good-sized specimens are sold at
about eight to ten dollars each ; at Paris, a good iolite, ten lines
long and eight and a half broad, was sold for one hundred and sixty
francs. When, a couple of years ago, the iolite was discovered by
Professor Mather, at Haddam, Connecticut, it promised to be a valuable
acquisition to American gems; but the supply was very scant, and its
original locality appears to be exhausted. Professor Torrey possesses a
fine seal, in the form of a cube, from that locality, which displays
its properties to the greatest perfection.
A
blue quartz is occasionally sold for iolite, but it may easily be
distinguished by its colors and hardness. Sapphire is considerably
harder than the iolite.
OPAL.
The
precious variety of this mineral was known to the ancients, and
received its name on account of the "play of colors which it has.
The-opal has a great many varieties, which are all considered more or
less gems, and find their application in jewelry; they will therefore
be treated separately. But, as general characters, it may now be
mentioned that opal scratches glass but slightly, while it is marked
by rock-crystal; it has a specific gravity of 2.06 to 2.11 ; it is
infusible before the blowpipe, but decrepitates and falls into
splinters; it also dissolves with borax. Opal consists of silica with
water, some oxide of iron, and sometimes alumina.
PRECIOUS OPAL.
This gem derives its name from the Greek word sig nifying the eye, for the ancients believed that this stone
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