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RUTILE
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This
mineral has luster, hardness, and power of resistance to solvents
sufficient to fit it for use as a gem, but ordinarily lacks
transparency and brilliancy of color. Rutile is oxide of titanium,
containing more or less iron. Its usual color is reddish-brown, passing
into black with a higher content of iron. The latter variety, known as
nigrine, gives, when cut, a stone closely resembling the black diamond
in appearance. The luster of rutile is adamantine, like that of the
diamond; but owing to its being rather opaque, its luster usually
borders on the metallic also. It is rarely sufficiently transparent to
make clear stones of any considerable size. At times, however, pieces
are found which cut into gems almost like the ruby. Rutile is the
mineral which usually forms the hair-like crystals penetrating quartz
and other minerals, and these often have a blood-red color. The
hardness of rutile is 6-6.5. Its specific gravity is high, often
enabling one to recognize it by simply taking it in the hand. It equals
4.2. Rutile is infusible before the blowpipe, and is unattacked by
acids.
What
are perhaps the finest rutile crystals known in the world come from
Graves Mountain, Georgia. Here long, splendent crystals are obtained,
which are objects of sufficient beauty to be worn uncut. It is
characteristic of rutile to form groups of crystals, each meeting the
other at an angle, so as to form a complete polygon. These objects make
natural ornaments also.
Rutile crystallizes in the tetragonal system. The cut stones are usually given the form of brilliants.
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137
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