272 A POPULAR TREATISE ON GEMS.
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probably he alluded to the emerald; for the same mineral is at the
present time called the emerald mother or matrix by jewellers. Prase
occurs massive and crystallized; it has a conchoidal fracture ; is
translucent on the edges; between vitreous and resinous in lustre ; and
of a garlic-green color, the cause of which is, that actinolite is
intermixed with the silex. It scratches glass ; has a specific gravity
of 2*66 to 2-8S, and is composed of silex, alumina, and oxides of iron
and manganese. It is found in Saxony, Tyrol, Styria, Hartz, and the
island of Elba. It is used for rings and pins ; also for snuff-boxes
and other jewelry, and is cut in cabochon, and set with a gold foil at
the base, by which its color is heightened, and rendered more
agreeable. It is used in mosaic works, for foliage; and likewise in the
mounting of rubies, in order to raise their-color. Prase does not stand
in great estimation; for although it assumes a very good polish, it
loses the same on long lexposure to the air, and grows spotty.
Avanturine.
This
mineral received its name from bearing a resemblance to a glass paste,
formerly manufactured in Italy. It is a brown or red quartz, which is
massive and translucent, or opaque; it has a resinous lustre, and its
fracture is splintry and uneven ; it is penetrated with gold or
brass-yellow glistening fissures, caused by the refraction of light,
or by innumerable mica leaves. It scratches white glass ; has a
specific gravity of 2'64 to 2-68 ; silex, with some alumina and water,
are its constituents.
The
avanturine is found in the Uralian mountains, Styria, Cape de Gata,
near Madrid, Nantes, Scotland, &cc. It is used for ring-stones,
ear-rings, and snuff-boxes. It is cut on a copper wheel, with emery,
and polished with rotten-
