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PRECIOUS STONES 179
Idocrase is derived from the Greek eido, to see, and krasis, mixture:
an allusion to the resemblance of its crystalline forms to those of
other species. The crystallization is di-metric, in modified square
prisms. Hardness, 6.5; specific gravity, 3.3 to 3.5. Cleavage
indistinct parallel to sides. Sub-transparent to opaque. It has been
found sufficiently transparent to cut for initial jewels. Doubly
refractive to a high degree. Composition: silica, lime, alumina, and
ferrous oxide mainly. Fuses under the blow-pipe with effervescence to a
yellow bead, and is attacked by acids. Color, yellowish brown to brown,
and red; sometimes green; in some varieties it is oil-green in the
direction of the axis, and yellowish green at right angles with it.
Streak uncolored. It is obtained in various parts of Europe and the
United States. A brown variety from Bohemia has been called " egeran."
Blue crystals, supposed to be colored by copper, have been named "
cyprine."
Willemite,
formerly called " troostite," is an anhydrous silicate of zinc found in
Belgium and New Jersey. It occurs in large hexagonal prisms with
rhombohedral terminations. Hardness, 5 to 5.5; specific gravity, 4 to
4.1. It does not take a high polish, but it is hoped that it may be
found sufficiently transparent for gem use. Lustre vitreous; opaque to
transparent. Color, gray, honey-yellow to apple-green, and a rich
brown. It consists of silica, 27.15 ; oxide of zinc, 72.85.
Williamsite
is a rich green variety of serpentine, used for decorative purposes. It
is translucent and frequently mottled. Hardness, 4; specific gravity,
2.6.
Wollastonite
is a tabular spar found in Canada, New York State, Pennsylvania, and
Lake Superior. Crystallization monoclinic, in distinct oblique
flattened prisms, to a fibrous structure; usually massive. Hardness, 4
to 5, brittle, and nearly as tough as jade. Specific gravity, 2.75 to
2.9. Lustre vitreous, inclining to pearly; translucent. Cleavage
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