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Classifications of Stones 13
A. Gems of the First Rank
Hardness,
between 8 and 10. Consisting of pure carbon, or pure alumina, or with
alumina predominating. Fine specimens of very rare occurrence and of
the highest value.
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B. Gems of the Second Rank
Hardness,
between 7 and 8 (except precious opal). Specific gravity usually over
3. Silica a prominent constituent. In specimens of large size and of
fairly frequent occurrence. Value generally less than stones of Group
A, but perÂfect specimens are more highly prized than poorer specimens
of Group A.
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C. Gems of the Third Rank
These
are intermediate in character, between the true gems and the
semi-precious stones. Hardness between 6 and 7. Specific gravity
usually greater than 2.5. With the exception
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