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
6|. 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—•
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.
