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Sec. IV, Ch. 35: Semi-Precious Stones, Tourmaline

Sec. IV, Ch. 35: Semi-Precious Stones, Tourmaline Page of 366 Sec. IV, Ch. 35: Semi-Precious Stones, Tourmaline Text size:minus plus Restore normal size   Mail page  Print this page
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Tourmaline.
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.
Sec. IV, Ch. 35: Semi-Precious Stones, Tourmaline Page of 366 Sec. IV, Ch. 35: Semi-Precious Stones, Tourmaline
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