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42
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PRECIOUS STONES
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lessness—a
fault seldom, if ever, met with in the trade. For example—a piece of
rock-crystal, chemically coloured, and cut to represent a ruby, might
appear so like one as to deceive a novice, but the mere application to
its surface of a real ruby, which is hardness 9, or a No. 9 needle,
would reveal too deep or powdery a scratch; also its possibility of
being scratched by a topaz or a No. 8 needle, would alone prove it
false, for the corundum group, being harder than No. 8, could not be
scratched by it. So would the expert go down the scale, the tiny
scratches becoming fainter as he descended, because he would be
approaching more nearly the hardness of the stone under test, till he
arrived at the felspar, Xo. 6, which would be too soft to scratch it,
yet the stone would scratch the felspar, but not zircon or andalusite,
7-1/2, or topaz, 8, so that his tests would at once classify the stone
as a piece of cut and coloured quartz, thus continuing what he would,
at the first sight, have suspected it to be.
The
standard stones themselves are much more certain in results than the
needles, which latter, though well selected and tempered, are not
altogether reliable, especially in the more delicate distinctions of
picking out the hardest of certain stones of the same kind, in which
cases only the expert judge can decide with exactness. Accurate in
this the expert always is, for he judges by the sound and depth of his
cut, and by the amount and quality of the powder, often calling the
microscope to his aid, so that when the decision is made finally, there
is never the least doubt about it.
Rapidly as these tests can be made, they are extremely reliable, and should the stone be of great value, it is
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