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14 GEMS AND PRECIOUS STONES IN THE
International
carat weighs.205 grams, equivalent to 3,168 grains troy. The
proportions according to quality of the entire South African yield are
as follows : First quality, eight per cent., second quality,
twenty-five per cent, third quality, twenty per cent., and the balance
bort, which is used for slitting gems, polishing diamonds, more
recently for saws, and ground into powder for use in the arts. An
impression seems to prevail that a diamond will not break if struck
with a hammer on an anvil, and several that were supposed to be good
specimens were broken in this way. While the diamond is hard, it is
also very brittle, and can be easily broken, and although every
substance from the hardness of feldspar up, including a cleavage or cut
diamond, will scratch glass, nothing but the natural edge of a diamond
crystal will cut it. j To determine whether a given specimen is a
diamond, the best test is to try if it will scratch corundum. If no
mark is produced, and if the specimen cannot be scratched by a diamond,
it is safe to assume that it is a diamond. It is well to make the trial
on a smooth or polished surface, otherwise the scratch will not be
perceptible.
The
occurrence of diamonds in the United States is chiefly confined to two
regions, geographically very remote and geologically quite dissimilar.
The first is a belt of country lying along the eastern base of the
southern Alleghanies, from Virginia to Georgia, while the other extends
along the western base of the Sierra Nevada and Cascade ranges in
northern California and southern Oregon. In both cases the mode of
occurrence has several marked resemblances. The diamonds are found in
loose material, among deposits of gravel and earth, and are associated
with garnets, zircons, iron sands, mona-zite, anatase, and particularly
with gold, in the search for which they have usually been discovered.
This resemblance is due altogether to the fact that these loose
deposits, in both regions, are merely the débris of the crystalline
rocks of the adjacent mountains, and therefore present a general
similarity, while the ages of the rocks themselves are widely
different. In the case of the South Atlantic States, the rocks of the
Blue Ridge and eastern Alleghanies are of ancient Archaen and Cambrian
ages, while in the western belt, the Sierra Nevada was not elevated and
meta-
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