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PRECIOUS STONES.
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crystallised
varieties are not the hardest, as Bort and Carbonado may slightly
exceed them. Crystals, too, vary somewhat in hardness in different
directions, probably more so in specimens showing strain than in
others, but there is a fairly constant difference in the direction of
the crystallo-graphic axes, the hardness being greater along the axes
than along the intermediate directions. Different faces of the
crystals, too, vary in hardness, and the exterior of a crystal is
usually harder than the interior. Crystals from various localities
differ : thus the Australian stones are remarkably hard, and the South
African ones relatively soft, especially before they have been long
exposed to the air.
With
regard to frangibility, it may be noted that the old idea that a
Diamond would splinter the anvil on which it was laid to be broken is
erroneous, for on the contrary it is a mineral easily reduced to powder
by a steel pestle and mortar.
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Crystalline Form, Occurrence, and Genesis.
Diamond
crystallises in the cubic system, and although the number of known
crystal forms is not great, there may yet be an enormous number of
planes on a natural crystal, since some of these forms, when complete,
may be composed of as many as forty-eight planes or faces. The " habit "
h 2
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