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Diamond

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76                                      GEMS.
Placed in the hydrostatic balance, it loses two-fifths of its weight, and this is the sole point of resemblance between it and the white topaz of Brazil, excepting the colour.
The different specific gravity of the Tarions diamonds under the same form is clearly produced by the different degrees of colouring, proceeding from the metallic oxides, which are heavier than the crystalline substance which contains them. It seems, however, that those contained in the American diamond are less pure, and therefore lighter.
One of the most beautiful qualities of the diamond is its power of refraction ; that of water is 0-785 ; that of the ruby, 0.739 ; that of the rock crystal, 0.654 ; that of the diamond, l-396.
The refraction of the diamond is single in the entire crystals ; when broken, it possesses double, but imper­fect refraction, in the thin layers.
The polarization of light gives in the topaz an angle of 31° ; in the Strass, 35° ; in the diamond, 22°.
The purest diamonds, that is, those of the finest quality, are quite devoid of colour ; in other words, when facetted, they reflect all the prismatic colours.
In the mines, however, these were only found in the proportion of one-fifth ; those of the second quality, and much less esteemed, are in the proportion of one-fourth, and are covered with a yellowish, greenish or bluish colour ; the remainder present the most varied colours.
The colouring principles are due to oxides and me-
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