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GEMS.
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the Latin verb micare, to
shine. Some kinds of mica are known "which have only one axis of double
refraction, attractive in some, repelling in others. There are even
some -which have two axes, and present varied degrees of divergence.
These peculiarities, which are easily perceived, on account of the
layers being perpendicular to the axis, indicate essential differences
of crystallization, and also different species. Their composition does
not offer less difference ; some contain lithia, others potash,
magnesia, or lime. Sometimes these substances are all united in it, in
varied proportions.
Black
and dark mica is found, having in it a great quantity of peroxide of
iron or of the corresponding oxide of manganese, and also of chromium.
In general magnesian mica is softer to the touch, and always less elastic than the others. That kind having a base of lithia is frequently called lepidolite, as
already remarked. Mica is often found crystallized, but only seldom
under the form of regular crystals, which seem hexagonal prisms or
oblique rhomboidrical prisms. Its specific gravity is 2.7. It is found
particularly in crystalline earths or in primitive rocks, and it may be
said to exist in every part of the earth, being found in granite and in
sand, in basalts and slates, in tufa, and in the latest deposits of
tertiary districts.
The colours of the different materials called mica offer every gradation of shade.
There is the downy mica, which takes its name from the resemblance it bears to down. This effect is pro-
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