action
and currents the cliffs of the shore are continually being worn down,
and the lighter and finer particles borne sea-ward, while those which
are heavier, either because of higher specific gravity, or of greater
resistance to erosion and decomposition, and hence larger, remain
behind. A continual concentration is thus going on which in time may
produce gem deposits of some extent. The area upon which such a
deposition may take place is, however, relatively narrow at any one
period, as compared with that afforded by streams, and hence few gems
are likely to be obtained from such sources. Labradorite and
hypersthene are obtained from deposits of this character upon the coast
of Labrador; chlorastrolite from the shore of Isle Royale; and agate
and thomsonite from beaches of Lake Superior. Hardly any other gem
minerals can be mentioned as so obtained, with the exception of amber,
which is gathered from the coast of the Baltic Sea. This, however, is
deposited not through its heaviness but its lightness, it being borne
upon the waves and tossed inland.
Passing
from the gravels in which gems are found to a consideration of their
original rock matrices, it may be said that rocks of the kind known as
metamorphic are more commonly than any others the home of the gem
minerals. Metamorphic rocks are those which have been changed by heat
and pressure, or chemical agencies, from their original condition. They
include crystalline limestones, quartzites, mica and hornblende
schists, gneisses, eclogites, etc. The rubies of Burmah, the emeralds
of the Urals, the diamonds of Brazil and the garnets of the Alps are
illustrations of gems which occur in this way.
Next
to metamorphic rocks those of an eruptive character afford the gem
minerals in the greatest abundance. Of these the acidic rocks, i. e.,
those containing a relatively large quantity of silica, such as the
granites, trachytes, rhyolites, and syenites, are the most prolific.
The coarsely crystallized form of granite known as pegmatite is
especially fertile in the gem minerals. The basic eruptive rocks, i.
e., those poor in silica, afford among gem minerals, chrysolite, some
garnet, some corundum, vesuvianite, and a few others. They are,
however, comparatively barren. The diamonds of South Africa occur in a
rock seeming to be of a basic eruptive character; but whether the
diamonds are of primary or secondary origin is not yet known.
Of
all the great groups of rocks those of sedimentary origin furnish the
fewest gems. Those which do occur in these are for the most part
probably derived from older eruptive rocks. Such is believed to be the
origin of the emeralds of Colombia, which are found in a bituminous
limestone of Cretaceous age. The opals of New South Wales, how-
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