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100
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PRECIOUS STONES
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of
the crystal, too, varies considerably, though most comÂmonly the
octahedral forms predominate (Fig. 9); twin crystals are often seen,
both of the contact and interpene-tration types. By suppression of
alternate faces of the
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octahedron,
forms of the regular tetrahedron arise. The plane of twinning is
parallel to the octahedron, and the twins are often flattened parallel
to the same form so as to have the general appearance of basal slices
of two triangular pyramids applied to one another. The faces are
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