prism
faces are also deeply grooved parallel to the vertical axis. The
vertical axis is polar—different crystal forms are present at opposite
ends—one termination is more obtuse than the other. Two sets of prism
faces occur—hexagonal prism and a trigonal prism, and terminated by
trigonal pyramids, and also rhombohedral faces. It also forms compact
or disseminated masses and divergent aggregates. It has a rounded
triangular outline. Cleavage—not well-developed. Fracture is
sub-conchoidal to uneven. Hardness = 7 to 7-1/2, not hard enough ta be
used in rings, but good enough for being used in brooches, pins, etc.
Gem tourmaline has a mean refractive index of 1-63, double refraction
0-020 and dispersion 0-016. It shows a wide range of colours—from
colourless to jet black and opaque; it may also show bright colours as
red, yellow, brown, green or indigo-blue. One and the same crystal may
show different colours. Zonal arrangement of colours may also be found,
e.g. parallel to the prism faces. Tourmaline has also strong
pleochroism or dichroism. In dark crystals the ordinary ray is
completely absorbed and the extraordinary ray is allowed to pass. So a
basal section is dark, whereas a section cut parallel to the vertical
axis will show some transmitted light and colour. Two such slices cut
parallel to the vertical axis could make a polarizing apparatus,
'tourmaline tongs'. When the slices are held parallel, light is allowed
to pass and when in crossed position, light is cut out. On account of
this property, in cutting the table, facet has to be parallel to prism
faces. It has also the property of developing pyroelectricity or
piezo-electricity due to its hemimorphic development. When a crystal of
tourmaline is changing in temperature, one end develops positive
electricity and the other end develops negative electricity. When
pressure is applied in the direction of elongation, a charge of
piezo-electricity is also developed. This property is taken advantage
of for
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