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INTRODUCTION.
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5
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The natural characteristics of gems are :
1st. The properties derived from the nature of the
substance itself; namely, hardness and specific gravity.
2nd. Those phenomena which are produced more
immediately by some external influence ; as light,
electricity, and heat.
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HARDNESS.
In
gems, hardness denotes not only tenacity, but also the greater or less
resistance which they oppose to being cut, scratched, or polished by
others ; thus, while the diamond from its small tenacity is easily
broken, and does not give out sparks under the action of the steel, it
is the hardest amongst gems, because it cuts all other bodies, without
being itself marked by them.
The
hardness of gems, considered as the attraction of cohesion—that is, in
the power they have of resisting the blows they ïeceive to break and
divide them mechanically—differs considerably in the same species,
according to the direction of the cutting, the surface of the stone on
which the trial is made, and its more or less perfect quality.
Hardness,
and not cohesion, is the basis on which general experience rests for
determining to what species a given gem belongs. For such a purpose,
practitioners adopt the file ; but the mineralogist rubs the stone
which is to be tried against another, beginning with the hardest gem,
which is the diamond,
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