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Ch. 7: Opal

Ch. 7: Opal Page of 311 Ch. 7: Opal Text size:minus plus Restore normal size   Mail page  Print this page
148                               PRECIOUS STONES.
basal face, or plane normal to the vertical axis, is only very rarely seen; crystals may be bighly modified by many crystal forms. The more purely crystallised varieties usually show a point of attachment at one end of the vertical axis. Twinned crystals are often seen.
In its origin, Quartz shows great variety. It is an original constituent of most volcanic rocks, but the kinds
we have to deal with more particularly are usually the result of solution of the disseminated mineral, or of the decomposition of silicates, and redeposition in larger masses. The wonderful groups of Rock Crystal found in fissures in the rocks of the Alps, originated in this manner. It is a common constituent of mineral veins, more often as a deposit from uprising heated waters, but sometimes as a later infiltration probably from above.
Ch. 7: Opal Page of 311 Ch. 7: Opal
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