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Hardness

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INTRODUCTION.
5
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.
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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