form of feldspar known as sunstone in appearance, but can easily be distinguished from it by its greater hardness.
The
best known localities for aventurine at the present time are the Ural
and Altai mountains in Russia. In the former it occurs in the vicinity
of Slatoust, in strata of mica schist, and in the latter not far from
Kolivan. The aventurine from the latter locality is cut into large
vases and dishes. One of the finest of these is to be seen in the
Museum of Practical Geology in London. This vase was presented by
Nicholas I. to Sir Roderick Murchison in recognition of his services in
investigating the geology of the Russian empire. Aventurine is said to
be highly regarded in "China, the imperial seal being always made from
it. India and several localities in Europe furnish aventurine; but none
of good quality has as yet been found in the United States. Owing to
the facility with which it can be obtained in masses it is not used
extensively except for making large ornamental objects. Together with
sunstone it can be quite successfully imitated in glass.
Crypto-crystalline quartz. The
crypto-crystalline (obscurely crystalline) varieties of quartz are
many. The following may be named as the most important: Chalcedony,
carnelian, sard, chrysoprase, prase, plasma, bloodstone, agate, onyx,
sardonyx, jasper, basanite, flint, and hornstone. The distinctions
between the different varieties are loose, and are differently stated
by different authorities. Some class agate, onyx, sardonyx, plasma, and
carnelian as varieties of chalcedony, while others consider chalcedony
a simple variety.
The chalcedonic varieties of quartz agree in having a fibrous structure and in being somewhat softer (hardness 6-1/2) and
somewhat lighter (specific gravity 2.6) than crystallized quartz. They
also break with more difficulty than quartz, being very tough. The
varieties differ among themselves chiefly in color.
Chalcedony has
a waxy luster, and is usually translucent rather than transparent. The
transparent forms are known as "Oriental," the translucent as "
Occidental" chalcedony. Common chalcedony has little color, shades of
gray and blue being the most common, although other tints occur. It
usually presents rounded surfaces which have grape-like, kidney-like,
or stalactitic forms. It occurs coating other rocks or minerals, or
lines cavities, or fills veins and clefts. It is never, so far as
known, deposited in any other way than by percolating waters. At Tampa
Bay, Florida, the waters containing chalcedony have penetrated corals
and preserved them, often giving forms showing the shape of the coral
outside and a cavity within. Throughout the " Bad Lands " of the West,
clefts in the hills are often filled with
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