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Ch. 6: Ruby, Sapphire, Spinel etc.

Ch. 6: Ruby, Sapphire, Spinel etc. Page of 295 Ch. 6: Ruby, Sapphire, Spinel etc. Text size:minus plus Restore normal size   Mail page  Print this page
124                     The Topaz.
occurs of a very large size without defects, and many of the large stones commonly sold as such are merely rock-crystal or quartz of a yellow colour, such as cairn­gorm and others, between which and the true topaz, however, there are many important differences, as a re­ference to the chapter on quartz will show.
The topaz belongs to the trimetric system of crystal­lization. Its cleavage is basal ; that is, it breaks with a flat surface, at right angles, with or across the prism. Its sperine gravity is 3·5 to 3.6; it is numbered eight in the scale of hardness, scratches quartz easily, and pos­sesses double refraction in a slight degree.
Its chemical composition is—
Monsieur St. Clair Deville has detected vanadium in the variety from the first locality, and he states that topazes may be formed artificially by the wet process. The crystals are usually hemihedral, which is a term used in mineralogy to signify (what its name infers) half-form ; that is, its angles are removed from the original form; for instance, the hemihedral form of the cube is the tetrahedron.
Ch. 6: Ruby, Sapphire, Spinel etc. Page of 295 Ch. 6: Ruby, Sapphire, Spinel etc.
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