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CYANITE
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(DISTHENE)
The
character of this mineral in its employment as a gem is indicated by
the derivation of its name, which is from the Greek word kuanos, meaning
blue. While cyanite at times presents other colors, such as gray,
green, black, and white, only the transparent blue variety is used for
gem purposes. This is often dark blue, resembling sapphire in color,
and cut stones may thus considerably resemble the latter. Like
sapphire, cyanite is largely composed of aluminum, but it is a silicate
of this metal instead of an oxide. The percentage composition of
typical cyanite is alumina 63.2, silica 36.8. Cyanite is not as hard as
sapphire, being 5-7 in hardness. One of its peculiarities is that the
hardness differs in two directions. If one of the broad blades of the
mineral be scratched in one direction a hardness of 5 is observed,
while in a direction at right angles to this the hardness will be found
to be 7. The name of disthene, by which cyanite is sometimes known,
refers to the above differences, it being derived from dis, twice, or of two kinds, and sthenos, strong.
Cyanite usually shows slight differences of color in different
directions. It has a marked cleavage, which somewhat interferes with
its use as a gem, cracks being easily started in this direction. Its
specific gravity is comparatively high, being 3.55-3.65, nearly but not
quite equal to that of sapphire. Its crystals belong to the triclinic
system.
Its
occurrence is usually in mica schists and gneisses, it being often
accompanied by garnet and staurolite. The decay of the mother rocks
leaves it in form of rolled pebbles, in which manner it occurs in
Russia, India, and Brazil. Cyanite from all these localities affords
good stones for cutting, Monte Campione, in the St. Gothard region of
Switzerland, furnishes some of the finest crystals known. Nearly all
that has been used for gem purposes in this country has been obtained
near Bakers-ville, North Carolina. The comparative softness of cyanite
is a bar to its extensive use as a gem, although in respect to color
and luster it is of a pleasing character.
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