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CRYSTALLINE STRUCTURE 17
to the vertical axis, which may vary in length, more so even than the other two.
The
fifth, the monoclinic—or clinorhombic, mono-symmetric, or
oblique—system, has also three axes, all of them unequal. The two
lateral axes are at right angles to each other, but the principal or
vertical axis, which passes through the point of intersection of the
two lateral axes, is only at right angles to one of them.
In
the sixth and last system, the triclinic—or anor-thic, or
asymmetric—the axes are again three, but in this case, none of them are
equal and none at right angles.
It
is difficult to explain these various systems without drawings, and the
foregoing may seem unnecessarily technical. It is, however, essential
that these particulars should be clearly stated in order thoroughly to
understand how stones, especially uncut stones, are classified. These
various groups must also be referred to when dealing witli the action
of light and other matters, for in one or other of them most stones are
placed, notwithstanding great differences in hue and character ; thus
all stones exhibiting the same crystalline structure as the diamond are
placed in the same group. Further, when the methods of testing come to
be dealt with, it will be seen that these particulars of grouping form
a certain means of testing stones and of distinguishing spurious from
real. For if a stone is offered as a real gem (the true stone being
known to lie in the highest or cubic system), it follows that should
examination prove the stone to be in the sixth system, then, no matter
how coloured or cut, no matter how perfect the imitation, the test of
its crystalline structure stamps it readily as false beyond all shadow
of
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