PHYSICAL CHARACTERISTICS OF PRECIOUS STONES.
WEIGHT AND MOLECULAR ACTION.
Specific gravity.—Every
one knows that two equal volumes of different substances have seldom
the same weight: a piece of lead, for instance, is much heavier than a
piece of wood exactly equal to it in size. If we find the weight of a
substance, and also that of an equal volume of another substance, selected as a term of comparison (distilled
water is the term that has been chosen), and if we divide the weight of
the first body by that of the second, we obtain a number which
expresses how many times, and fractions of times, the body considered
is more or less heavy than that to which it is compared. The number
thus obtained is its specific gravity.
In
the case of precious stones it is a characteristic of extreme
importance, for it is frequently the means by which the difference is
detected between stones that the eye might easily confound. In this
way, for example, the diamond can be at once distinguished from the
zircon, the specific gravity of the former being 34., and that of the
latter 4-4.
Hardness.—We should be careful not to fall into
the very frequent error of confounding the quality
of hardness with that of resistance to crushing or
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