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14 A Book of Precious Stones
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of
turquoise, silica is a prominent constituent of all these stones. Value
usually not very great; only fine specimens of a few members of the
group (cordierite, chrysolite, turquoise) have any considerable value.
Specimens worth cutting of comparatively rare occurrence, others fairly
frequent.
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2. Semi-Precious Stones
These have some or all of the distinguishing characters of precious stones, but to a less marked degree.
D. Gems of the Fourth Bank
Hardness,
4-7. Specific gravity 2-3 (with the exception of amber). Colour and
lustre are frequently prominent features. Not as a rule perfectly
transparent: often translucent, or translucent at the edges only. Wide
distribution. Value, as a rule, small.
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