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TITANITE
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(SPHENE)
Titanite
is one of the few minerals which possess, like diamond, an adamantine
luster. This luster gives to gems cut from titanite a rich effect, but
they lack depth of color and hardness sufficient to make them stones of
the first rank.
Titanite
occurs in numerous colors, brown, yellow, and green being the most
common and characteristic. Stones cut from these are usually distinctly
pleochroic, showing red and yellow in different directions,
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while
in one direction they may be colorless. Only the transparent pieces are
cut for gems. They resemble chrysoberyl, topaz, garnet, or chrysolite,
in appearance. Their hardness is 5 to 5-1/2, somewhat below that
essential for a good wearing gem. The specific gravity of titanite is
3.4 to 3.55. In composition it is a titano-silicate of calcium, the
percentages being, silica 30.6, titanium dioxide 40.8, lime 28.6. It
is fusible before the blowpipe to a colored glass. It is attacked by
sulphuric and hydrofluoric acid. It crystallizes in the monoclinic
system, the crystals often having the shape of a wedge, whence the name
sphene, from the Greek sphen, a wedge, by which the mineral is often known.
The
finest transparent crystals of titanite come from Switzerland, being
generally of a yellowish-green color. Kunz mentions crystals of
titanite from Bridgewater, Bucks County, Pennsylvania, over an inch in
length, which afford fine greenish-yellow or golden stones, weighing 10
to 20 carats.
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138
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