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at
equal distances. Each face is an equilateral triangle. Diamond and
spinel are gem minerals which often exhibit crystals of this form The
dodecahedron is bounded by twelve faces, each of which meet; two of the
axes at equal distances, and is parallel to a third axis Each face is a
rhomb. Garnet quite commonly crystallizes in this form as well as in
that of the next type, the trapezo-hedron. The trapezohedron is bounded
by twenty-four faces, each of which is a trapezium. Each face
intersects one axis at the unit length, and meets the other two axes at distances greater
than unity. The form bears some relation in appearance to the
octahedron, if it be imagined that three faces of the trapezohedron
occupy the place of one face of the octahedron. In other systems than
the isometric, the
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simplest
and in general tne most commonly occurring torms are prisms and
pyramids. Prisms are forms whose faces are parallel to the vertical
axis, while they meet the lateral ones; pyramids are forms whose planes
meet all three of the axes.
In
the hexagonal system prismatic and pyramidal faces occur in multiples
of three, while in the tetragonal, orthorhombic, etc., systems,
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they
occur m multiples 01 two. Ihus a crystal 01 zircon may be distinguished
from one of quartz, for example, by the fact that on the former four or
eight similar prismatic faces may be counted, on the latter, three or
six.
Substances
vary considerably in their tendency to form distinct crystals, or even
to crystallize. Quartz, in the form of rock crystal and amethyst, is
generally found in distinct crystals, while agate, chalcedony, etc.,
although crystalline, and of the same composition, never form separate
crystals. . Such substances as opal, turquois,
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obsidian,
and obviously those of organic origin, such as amber, jet, pearl, and
coral, never crystallize, or possess regular external form. Such
substances are termed amorphous. As a rule, gem minerals are
those tending to occur in distinct crystals, since crystallization
usually favors transparency and purity of substance.
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40
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