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Ch. 3: Physical Properties of Gem Stones

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PRECIOUS STONES.
this accounts for much of the variation. For example, Quartz in its crystalline forms of Rock Crystal, Amethyst, etc., has the standard hardness of 7'0, but the cryptocry-stalline varieties Agate and Chalcedony have a hardness of only 6-5 in some cases.
VII. Cleavage.
Minerals which occur in crystalline masses or in actual crystals (vide infra) have a very definite internal arrange­ment of the molecules composing them, causing the mineral to have different properties in different directions, whereas
Ch. 3: Physical Properties of Gem Stones Page of 311 Ch. 3: Physical Properties of Gem Stones
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