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220 PRECIOUS STONES.
strongly
dichroic. Emerald and Dioptase differ from Diopside in colour. A glass
imitation may at once be known by the single refraction.
327. Spodumene.
Spodumene
is another mineral belonging to the Pyroxene group, but until
comparatively recent times it was of no importance as a precious stone.
However, in 1879 Mr. W. E. Hidden discovered a beautiful variety, which
has been named in his honour ; and more recently still another variety
has been found in California, and named by Dr. Baskerville after Dr. G.
F. Kunz, the author of " The Gems and Precious Stones of North America."
The
colour of the ordinary variety of Spodumene is white, grey, yellow, or
rarely somewhat violet; Hiddenite varies from yellow-green to
emerald-green, and it is from specimens of the latter colour that the
gem stones are cut; Kunzite is found in various shades of pink, violet,
and blue. The mineral is transparent to translucent, the lustre is
vitreous. It is doubly refracting, the greatest and least indices for
the D line being 1*68 and 1"65. It shows strong pleo-chroism, and the
variety Kunzite has a marked phosÂphorescence induced by exposure to
X-rays and to the emanations of radium. The specific gravity is 3"15 to
3"20, and the hardness is 6-1/2 to 7. In crystalline form it is
monosymmetric, and the habit is usually prismatic. The crystals show
vertical striations, and Hiddenite often shows etched markings. There
is a perfect cleavage parallel to the prism faces. The crystals of the
ordinary variety are often very large, up to 4 feet long and 1 foot
across. HidÂdenite occurs in slender crystals of 1-1/2 to 2 inches in
length.
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