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Ch. 5: Quartz

Ch. 5: Quartz Page of 296 Ch. 5: Quartz Text size:minus plus Restore normal size   Mail page  Print this page
150
PRECIOUS STONES.
other three, and thus we have the crystal repre­sented by Fig. 69.
In other cases the crystals are not terminated by pyramids, but by ridges, as in Fig. 70. In this case the form of the crystals is greatly altered, and the regularity, to a certain extent, disappears.
If in the regular crystal (Fig. 68) we suppose the prismatic part to be diminished little by little with­out the form being otherwise changed, then, when
the prismatic portion is quite removed, and the pyramids are applied base to base, the crystal re­presented by Fig. 71 is obtained. It is a dode­cahedron, all of whose faces are equal, and bounded by isosceles triangles.
Crystals of this form occur in the midst of the gypsum which accompanies the ophites of the Pyre­nees. Numerous examples of perfect purity occur also in the gypsum beds of Provence, which belong to the trias formation.
Ch. 5: Quartz Page of 296 Ch. 5: Quartz
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