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72
GEMS.
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It
was much used in jewellery at the beginning of this century ; I do not
know if it was used previously, nor have I ever seen a Chrysoprase, cut
or engraved, which bore the stamp of antiquity.
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XXXII.
ROCK CRYSTAL.
This
name was given to the transparent, white vitrified quartz, probably
because the finest pieces of this mineral are found on the highest
mountains. It is an oxide of hydrate of silica, and crystallizes in
hexahedral prisms ; but this form is only met with in the perfect state
in crystals which are isolated and detached from their matrix.
The rock crystal is so hard that it scratches agates, and emits sparks when struck by the steel. Its specific gravity is from 2-6548,
and it is composed of silica and oxygen in equal parts, besides a very
small quantity of alumina and lime. It resists fire and acids well ;
has double refraction, and cleavage.
It
is found in every part of the globe ; the Alps, Sardinia, Ceylon,
Haiti, the Indies and Hungary possess it. Prom Madagascar and Brazil it
is taken in the largest masses.
However
pure this mineral may be, the lamina which compose it are not all of
the same hardness. This has been proved by various experiments ;
nevertheless, the
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