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PRECIOUS STONES.
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harder
; coloured Spinels are not dichroic. Quartz, Phenakite and Tourmaline
are less dense, and Aquamarine shows yellowish and sky blue images in
the dichroscope, while blue Topaz shows colourless and green images.
Corundum is more dense. Fluor Spar is very much softer and shows single
refraction. Topaz when yellow shows pale yellow and yellow-red
dichroscope images ; burnt Topaz shows cherry red and honey yellow.
Further, Topaz is easily electrified.
398. Andalusite.
Andalusite,
which is also known as Chiastolite, though this term is now rather
confined to a variety, is a widely distributed mineral, occurring as a
result of the metamor-phism of argillaceous rocks by hydro-thermal
action. The walls of the old farm house in Cumberland in which these
words are penned are thickly studded with crystals of it, but they are not of
gem quality. The only form used as a gem is found in Brazil in the
district near Novas Minas, in gravel. These specimens are of green or
yellow-brown colour. The mineral here is transparent, and has a rather
poor vitreous lustre. It shows a weak double refraction, the greatest
and least indices being 1'643 and 1"632. It, however, shows a very
marked pleochroism ; when viewed along the vertical axis the colour is
a rich red, while in the direction of the horizontal axes the colour in
both cases is green, but of slightly different tint. It is infusible
before the blowpipe. Its specific gravity is 3-16 to 3-19. The mineral is brittle and shows a subconchoidal fracture, and has a hardness of 7-1/2. There is a distinct cleavage parallel to the face of the rhombic prism. The streak is colourless,
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