when perfectly transparent and pale in colour may be cut into a lustrous gem-stone. Its specific gravity is nearly 7, and its hardness about 6½. Rutile, when of a transparent red colour, yields a cut stone of very high refractive index, and presents a lustre almost metallic on the polished surfaces. But Rutile is, perhaps, best known in the form of acicular crystals, red or reddish-brown in hue, which, when penetrating rock crystals, constitute the Veneris crinis of Pliny. Of anatase we need only say that some of the indigo-blue transparent and splendent crystals from Brazil have been mounted, either in their natural forms or step-cut, in jewellery. They have the form of beautiful low octahedra belonging to the tetragonal system: their specific gravity is about 4·86.
Diopside.
This mineral has been occasionally cut as a gem-stone; it presents a close resemblance to dull green tourmaline or epidote. Its hardness, however, does not exceed 6. The specific gravity of a fine cut diopside was 3·306. Its colour is due to ferrous oxide. It contains about—
Silica | 54 | Magnesia | 18 |
Lime | 24 | Ferrous oxide | 4 |
Apophyllite.
Apophyllite can hardly be regarded as a gem-stone, its softness causing its rapid abrasion. The hardness of apophyllite does not exceed 5; its specific gravity is 2·335; its colour varies from nearly transparent white to grey, yellowish, greenish, and flesh red. This mineral crystallises in the tetragonal system, the forms assumed being usually an octahedron, with the solid angles truncated; the basal planes have a decided pearly lustre, the other faces are merely vitreous.
Apophyllite is found in amygdaloid and related rocks, also in mineral veins, as at the silver mines of Andreasberg in the Hartz.