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—
