copper
in some sedimentary rocks, and is thus widely distributed; yet
specimens suitable for ornamental use are not at all common, most of
them being found in the Ural Mountains ; thus at Nizhni Tagilsk, one
mass was found measuring some 160 square feet, on the upper surface.
Now but few of the mines produce it in pieces large enough to cut. On
the west coast of Africa, Bembe has yielded fine pieces. In Australia,
the Cobar mines in New South Wales and the Burra-Burra mines in South
Australia produce good specimens. In the United States, the Humming
Bird mine in Arizona is a noted locality.
In composition it is a basic cupric carbonate, CuCo3, Cu(0H)2,
containing 72 per cent, of oxide of copper. The darkening seen on
heating is due to the driving off of water. It is easily reduced to
metallic copper, hence is a valuable ore of that metal when occurring
in sufficient quantity.
A
few intagli in Malachite are known; now its chief ornamental use is for
such articles as vases, letter weights and other smaller objects ;
often it is used as a veneer and for inlaying; rarely is it cut as a
gem stone; when it is, it is usually en cabochon, more rarely in steps.
The Felspars.
The
Felspar Group contains several closely related species which are of
secondary importance as precious stones. The individual members closely
resemble one another in crystalline form, cleavage, hardness and
specific gravity, and to some extent in origin, though some are
characteristic of one type of rock, and some of another. All we are
concerned with belong to the triclinic system, with one exception, that
being Orthoclase, In all, the common
P.s,
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