Gems Notes and Extracts Augusto Castellani, Mrs. John Brogden 1871

76 GEMS.Placed in the hydrostatic balance, it loses two-fifthsof its weight, and this is the sole point of resemblancebetween it and the white topaz of Brazil, excepting thecolour.The different specific gravity of the various diamondsunder the same form is clearly produced by the differentdegrees of colouring, proceeding from the metallicoxides, which are heavier than the crystalline substancewhich contains them. It seems, however, that thosecontained in the American diamond areless pure, andtherefore lighter.One of the most beautiful qualities of the diamond isitspower of refraction ;that of water is 0*785 ;that ofthe ruby, O739 ;that of the rock crystal, 0*654 ;that ofthe diamond, 1'396.The refraction of the diamond is single in the entirecrystals ; when broken, it possesses double, but imperfectrefraction, in the thin layers.The polarization of light gives in the topaz an angleof 31 ;in the Strass, 35 ;in the diamond, 22.The purest diamonds, that is, those of the finestquality, are quite devoid of colour ;in other words,when facetted, they reflect all the prismatic colours.In the mines, however, these were only found inthe proportion of one-fifth ;those of the second quality,and much less esteemed, are in the proportion of onefourth,and are covered with a yellowish, greenish orbluish colour ;the remainder present the most variedcolours.The colouring principles are due to oxides and me-