CLEAVAGE
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These
cleavage planes help considerably in the bringing of the stone to
shape, for in a broad sense, a finished cut stone may be said to be in
the form in which its cleavages bring it. Particularly is this seen in
the diamond " brilliant," which plainly evidences the four cleavage
planes. These cleavage planes and their number are a simple means of
identification of precious stones, though those possessing distinct and
ready cleavages are extremely liable to "start" or "split" on these
planes by extremes of heat and cold, accidental blows, sudden shocks
and the like.
In
stones possessing certain crystalline structure, the cleavage planes
are the readiest, often the only, means of identification, especially
when the stones are chemically coloured to imitate a more valuable
stone. In such cases the cleavage of one stone is often of paramount
importance in testing the cleavage of another, as is seen in the
perfection of the cleavage planes of calcite, which is used in the
polariscope.
It
sometimes happens, however, that false conditions arise, such as in
substances which are of no form or shape, and are in all respects and
directions without regular structure and show no crystallisation even
in the minutest particles ; these are called amorphous. Such a
condition sometimes enters wholly or partially into the crystalline
structure, and the mineral loses its true form, possessing instead the
form of crystals, but without a crystalline structure. It is then
called a pseudomorph, which is a term applied to any mineral which,
instead of having the form it should possess, shows the form of
something which has altered its structure completely, and then
disappeared. For
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