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 Many hard minerals may, however, be easily broken, fractured, or chipped, though they cannot be scratched: a very hard stone may be a very fragile one. Emeralds, zircons, and diamonds have often been ruined by a fall or a blow.

The scale of hardness adopted for minerals was devised by Mohs. Fragments of transparent minerals, which may be conveniently mounted in handles, are applied in succession to the stone under examination, so as to attempt to scratch its surface. When the stone neither scratches nor is scratched by any member of the scale, the hardness of the two stones is the same. When it scratches the softer, and is scratched by the harder of the two test-stones, some notion of its position between them may be gained by passing all three specimens, with slight pressure, over the surface of a fine, clean, hard file, one end of which rests upon the table, and noting their different degrees of resistance to abrasion and the sounds produced. In chapter vii. of this book will be found, under the description of each kind of precious stone, numbers which nearly represent the average hardness of good specimens of the several sorts according to the common mineralogical scale, which is

A list of the degrees of hardness of a considerable number of different gem-stones will serve to show their relative positions with regard to this scale. Although this character of hardness cannot be extensively used in the discrimination, of cut and polished gemstones, yet it is sometimes available even in the case of such specimens when unmounted, the " girdle " of the stone offering a suitable surface for a trial of hardness.