opalescent, and attributes the phenomenon to the presence of a hydrocarbon, which he named " Tiffanyite."
In
the quality of transmitting light, precious stones are divided into
four kinds,—opaque, as jasper; translucent, as the opal and carnelian;
semitransparent, as rose-quartz; transparent, as the diamond, etc. Not
all stones of the transparent varieties are transparent, however.
Carbonado and bort, which are forms of diamond, are opaque and
semitransparent respectively. Many crystals of ruby, emerald,
tourmaline, and others are almost opaque. Nature brings but little of
her handiwork to an ideal condition. Some stones, like the sphene, are
seldom found clear enough to cut as gems. Probably not one carat in ten
thousand emerald crystals taken from a mine in North Carolina was
transparent.
In
some respects, precious stones are constant; they resist or are
subject to heat or acids after their own invariable fashion. The
diamond is always infusible, but at a great heat will burn to dioxide
of carbon. Many stones are infusible before the blow-pipe, but melt
with a flux. Heat changes the color of the amethyst and topaz. The
beryls melt with salts of phosphorus. Rock crystal is soluble in
fluohydric acid. Obsidian melts under the blow-pipe. And although
crystals of the same mineral may occur with many modifications, the
prisms longer or shorter, thinner or thicker, some planes enlarged,
Others obliterated,—whatever the distortion, the angles remain
constant, the inclination is the same.
As
the colors of different stones are often identical, it is sometimes
difficult to decide, after cutting, what they really are, especially
when the softer stone is finely cut. Even the tests for hardness and
specific gravity are apt to fail in inexperienced hands, as the
processes require experience to arrive at the exact results necessary
where distinctions are so fine. An instinctive judgment which rarely
errs comes with long acquaintance. There is a certain hard look which
distinguishes a yellow sapphire from a topaz, or a purple sapphire
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