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Ch. 1: Precious-Stones

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ORIGIN, PROPERTIES, CLASSIFICATION, ETC.             15
the value of decorative stones, as in the diamond, which probably affords the best illustration of this quality ; but there is a
wide difference in the degree of this power possessed by most
gems, which receives different names, according to its strength
or feebleness. They are transparent, when objects are distinctly seen through them ; translucent, when light passes
through, but no objects are seen ; and opaque, when ho light
is transmitted. Some transparent gems become more or less
opaque when seen in certain directions. Writers on stones
sometimes use the first and second terms indiscriminately, calling a mineral transparent when it is only translucent. The
ancients accounted for the lustre and transparency of the diamond by supposing it was congealed water.
Hardness in precious stones is of great importance, since it
protects them from injury, renders them capable of a high
polish, and fits them for testing this quality in other species of
minerals. The property of hardness does not mean the power
of resisting crushing weight, since a very hard mineral may be
very brittle ; nor does it depend upon the tenacity with which
the particles cohere, or its infrangibility. since the hardest
stones, like the diamond, may be easily broken by a fall or a
blow ; but it implies the quality of resisting the action of a
point,—as of a needle, — or the difficulty of being scratched by
any softer substance.
The brilliancy and fire, or play of colors, are, to a certain
extent, influenced by the hardness of the substance, though not
in all cases, as in the opal. The diamond will not yield to any
other stone, but will scratch all others ; hence, it is ranked as
the hardest gem. The sapphire will resist quartz, proving the
latter to be the softer. The scale of hardness established by
Mohs ranges from i, the softest, to io, the hardest, a place
assigned to the diamond alone. Some of the best known
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