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Tourmaline.
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The
Tourmaline passes under a variety of mineralogieal names, according to
the colour which it presents. The red varieties are known as Rubellite, the blue as Indicolite, and the clear and colourless crystals as Achroite ; while the common black Tourmaline is still distinguished by the old German name of Schorl.
It
often happens that the colour is not constant throughout the stone, so
that one part may be green, while another portion of the same crystal
may be decidedly pink. An American variety is notable for presenting a
central kernel of red colour, surrounded by a zone of lively green, and
as such crystals are usually three-sided prisms, they offer, when cut
across, a triangular or heart-shaped section, with the pleasing effect
of a red centre fringed by a green border.
Tourmaline
possesses double refraction, and polarizes light perfectly : hence it
is used by opticians in the construction of polariscopes. Its dichroism
is very pronounced, and may be often recognised without the aid of an
instrument.
Tourmaline,
in common with many other Precious Stones, develops electricity under
friction. Many Tourmalines also acquire electric properties when
heated—one end of the crystal becoming positive and the other negative.
This phenomenon is known as Pyro-electricity. It is connected
with the curious form of most of the crystals, their two extremities
exhibiting different faces. This peculiarity of shape is termed hemimorphism, since
half of the crystal presents one form, and half another. When the
temperature of a hemimorphic crystal is either raised or lowered, its
electric equilibrium is disturbed, and polarity developed ; so that the
condition of the crystal may then be compared with that of a magnet.
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