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46 PRECIOUS STONES.
VIII. Crystalline Form.
The
greater number of bodies of definite chemical composition, including
minerals and therefore gems, occur in " crystals " or are "
crystalline." This leads us to the conclusion that the minute groups of
chemical atoms are arranged in some definite way. Each substance that
crystallises has its own definite geometrical form; this might not
appear to be so at first sight, for if we took a group of crystals of
Fluor Spar, for instance, gathered together from many different
localities, the specimens would seem at first anything but similar, yet
if we come to measure the angles between the different planes or
"faces," we should find that a good many possessed planes exactly at
right angles to one another; if the six possible faces of this kind
were all equally developed, we should see the specimen was in the form
of a perfect cube—all the six faces together belonging to the "form" of
the cube; but they might not be equally developed and the resulting
figure might be a parallelepiped, or rectangular solid figure (with
opposite sides equal necessarily) having adjacent faces of different
sizes. Still such faces would be parallel to the faces of a perfect
cube and, therefore, would be, crystallographically, identical. Again,
amongst the Fluor Spar crystals we might find many that showed the
solid angles of the cube truncated by a plane having the form of an
equilateral triangle. Since parallel planes are identical in a crystal,
we might imagine these triangular faces moved inwards towards the
centre of the crystal until they were, all eight of them (one at each
corner of the cube), equally distant from that centre; there would then
obviously be no face of the cube left, but a symmetrical eight-faced
solid
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