concussion.
There are minerals that may be crumbled between the fingers, and that
are yet none the less hard. The hardness of a substance, according to
the definition of Delafosse, is "the resistance which it opposes to
the' action of a point like that of a steel needle which may be drawn
across it, or to the angular part of another mineral passed with
friction over its surface."
Hardness
is an indispensable quality of gems. If a stone were not very hard, the
continual friction to which it is subjected would very soon destroy its
polish; and with the polish, transparency, brilliancy, fire—all that
constitutes its value—would vanish.
It
is owing to this quality of hardness, added to the unchangable nature
of their substance, that stones, cut perfectly by Egyptian artists
thousands of years ago, have reached us intact; and give us the most
interesting proof of the progress in arts and civilization which had
been attained in those remote periods.
Fusibility.—Fusibility
is the property which solid bodies possess of passing into the liquid
state, when they are subjected to a sufficient temperature.
For
precious stones in particular the point of fusion is lower in
proportion as the composition of the stone is more complex. Thus the
diamond, a simple body, is absolutely infusible. The ruby, the sapphire, the topaz, binary bodies, can only be
