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gem
material formed contemporaneously with the original rocks; sedimentary
rocks in some areas also contain gem minerals of detrital origin. Such
rocks when crushed, powdered and separated, yield small quantities of
gem material. The diamonds of India and Brazil and some portions of
Africa are detrital and occur in sedimentary rocks of pre-Cambrian age.
But diamond-bearing conglomerates of Triassic age are also found in
the Belgian Congo and Angola.
(c) Metamorphic rocks.—Sedimentary
or igneous rocks, when buried deep in the earth, are generally
subjected to great pressure and heat. They are exposed later by
denudation by the removal of the cover. In some of them gem minerals
have been found, e.g. corundum and garnets which have high density,
with other minerals such as kyanite, staurolite and beryl. Ground water
when circulating through cracks and crevices may cause secondary
changes to take place yielding a few new minerals, which, may have had
their origin in the country rock, or from outside or by interaction of
two sets of materials. A few minerals are formed in this way, e.g.
agate, amethyst, Hungarian opal, Persian turquois. There are also
secondary minerals in rocks of Tertiary age.
It
has been found that the same mineral may be formed in nature by more
ways than one. Corundum and beryl may be of igneous or metamorphic
origin, emerald in limestone or secondary origin. Quartz and its
varieties are formed under varying conditions. Though gem-stones have
been mentioned as occurring in rocks of all ages, it is important to
note that their occurrence is exceptional. Even if the stones are found
sparingly in the rock, it may not pay to break the rock for the
gem-stones. Nature, as already referred to, has in many cases done this
work for man by steady work through countless ages by the processes of
denudation. The material broken up is carried by running water and
sorted on the way—lighter
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