CASSITERITE, ANATASE, PYRITES, HEMATITE
CASSITERITE
Though usually opaque, this oxide of tin, corresponding to the formula
SnO_{2}, has occasionally, but very rarely, been found in small,
transparent, yellow and reddish stones suitable for cutting. The lustre
is adamantine. The refraction is uniaxial in character and positive
in sign, the ordinary index being 1·997 and extraordinary 2·093. The
specific gravity is high, ranging from 6·8 to 7·1. The hardness is on
the whole less than that of quartz, being about 6 to 7 on Mohs’s scale.
ANATASE
This mineral, which is one of the three crystallized forms of titanium
oxide, TiO_{2}, occurs often in small, brown, transparent stones which
occasionally find their way into the market. The lustre is adamantine.
The refraction is uniaxial in character and negative in sign, the
extraordinary index being 2·493 and ordinary 2·554. The specific
gravity varies from 3·82 to 3·95, and the hardness is about 5½ to 6 on
Mohs’s scale.
PYRITES, HEMATITE
These two metallic minerals were employed in ancient jewellery. The
former, sulphide of iron, FeS_{2}, is brass-yellow in colour, and has
a specific gravity 5·2, and hardness 6½ on Mohs’s scale. It is found,
when fresh, in brilliant cubes. The latter, oxide of iron, Fe_{2}O_{3},
has a black metallic lustre, but, when powdered, is red in colour—a
mode of distinguishing it from other minerals of similar appearance.
Its specific gravity is 5·3, and hardness 6½ on Mohs’s scale. In modern
times it has been cut in spherical form to imitate black pearls, but
can easily be recognized by its greater density and hardness. Hematite
is used for signet stones, often with an intaglio engraving.