Phenacite
affords transparent, colorless gems of a brilliant vitreous luster.
They are usually cut as brilliants, and stand between the diamond on
the one hand and rock crystal on the other in the amount of •'fire"
they display. Phenacite has stronger double refraction than quartz, and
a higher index of refraction. It is far, however, from equaling the
diamond in these properties. In fact, it resembles quartz so much that
it was not until 1833 recognized as a distinct species. The name of
phenacite, from the Greek phenax, a deceiver, was given to it
because of this resemblance. Like beryl and euclase, phenacite is a
silicate of glucinum. Its percentage composition is, silica 54.45,
glucina 45.55. It is infusible before the blowpipe, and can be
distinguished chemically from quartz by putting a drop of cobalt
nitrate on a heated fragment and then reheating. The fragment turns
blue if phenacite ; if quartz it remains black. Phenacite is harder
and slightly heavier than quartz, its hardness being 7.5-8, and its
specific gravity 2.97-3. It crystallizes in the rhombohedral division
of the hexagonal system. The gems are usually obtained from crystals.
Phenacite
is not a common mineral, and nearly all that has been cut for gems has
come from two localities, Takovaya, near Ekaterinburg, Russia, and Mt.
Antero, Chaffee County, Colorado. The first locality furnishes the
finest and largest stones, some of them weighing thirty to forty
carats. They occur together with emerald and chrysoberyl in mica
schist. The Mt. Antero phenacite is found at an elevation of 14,000
feet, and is obtainable for only a short period during the summer on
account of the abundant snows of the region. This locality affords
smaller gems than the Russian, and owing to the demand for them as
mineralogical specimens, few are cut. They usually occur implanted on
quartz, beryl, or feldspar. Some good phenacite has been found on Bald
Mountain, near North Chatham, New Hampshire, in a granite vein and near
Florissant, Colorado. Besides being colorless, phenacite may exhibit
pale rose and wine-yellow colors.
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