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HYPERSTHENE AND BRONZITE
These
minerals, belonging to the pyroxene group, are employed in jewelry
when, on account of a fibrous structure or a regular arrangement of
inclusions, they exhibit a chatoyant effect. They do not afford
transparent stones, but are cut en cabochon to make cat's-eyes and
give other schillerizing effects.
The
color of hypersthene is usually a dark bronze, so opaque as to approach
a metal in luster. The light which plays over it is copper-red in
color, and very brilliant in a good stone. The cause of the chatoyancy
is supposed to be countless crystals of the oxide of titanium, known as
brookite, which are arranged in regular order in the stone. The stone
must be cut with reference to the direction of these in order to give
the chatoyant effect.
The
hypersthene used for this purpose comes almost exclusively from the
Island of Paul on the coast of Labrador. Here it occurs together with
labradorite as shore pebbles, and it may also be quarried from
neighboring cliffs. For cutting, a sound piece without flaws must be
used, and it is often necessary to break a number of fragments before a
suitable one can be found. Yet the supply of material is so abundant
and the demand comparatively so limited that the stones do not command
a high price.
The
hardness of hypersthene is 5-6; its specific gravity 3.4-3.5. It fuses
before the blowpipe to a black enamel, and is partially decomposed by
hydrochloric acid. Its name comes from two Greek words meaning very
tough. It is a common constituent of eruptive rocks, usually in small
crystals. Its system of crystallization is orthorhombic. In
composition it is a silicate chiefly of iron and magnesium.
Enstatite
resembles hypersthene in composition and properties, and its limited
use in jewelry is to furnish " cat's-eyes " of a green color. The
chatoyant effect is due usually to a fibrous structure. The principal
locality for this variety is near Harzburg in the Harz. Schillerizing
bronzite is found in a few localities in this country.
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