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PRECIOUS STONES. 45
an
amorphous substance is similar in all directions. One of the phenomena
noticed in crystalline bodies is that the coherence in certain planes
is weaker than in other planes. There may be one or several such planes
of weaker cohesion, and this leads to the substance dividing along such
planes with more or less facility ; such a plane is known as a cleavage
plane, and as it always bears a definite relation to the crystalline
form of the mineral, it is of great use in identification when the
crystal faces are destroyed, or when the form of the crystal is very
complex. From the point of view of the gem cutter, these " cleavages"
are of great assistance when bringing the rough gem into approximately
the shape the finished stone is to assume. 'Ibis is particularly so in
the case of the Diamond, for it has four such cleavage planes which
tend to divide the mineral into the form of the regular octahedron,
from which form the commonest type of cut Diamond, the " brilliant,"
is readily derived. Adularia, a variety of Orthoclase Felspar, and
Topaz are two other precious stones which show cleavage well. Calcite
has a very perfect cleavage in three directions, tending to divide it
into rhombs, a fact of great help in making some of the instruments
used in the optical examination of precious stones, such as the
polari-scope and dichroscope.
A
cleavage plane may be distinguished from an even fracture, for the
former being a true plane, another such plane exactly parallel to it
can be easily produced.
It
must be borne in mind that a gem which possesses a distinct cleavage is
more liable, other things being equal, to be broken or damaged by
sudden changes of temperature.
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