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A stone of many colours is tourmaline; it was introduced into Europe from India in 1703 and its name is adapted from turmali, its Cingalese name. Tourmaline is a widely distributed mineral, and its transparent coloured varieties, used as gem stones, have attained a considerable popularity. The vogue of the tourmaline has increased since it was discovered in 1820 on Mount Mica near Paris, Maine. The tourmaline has also been found in Massachusetts, California, and New York State. Its principal sources are Ceylon, Burma, Brazil, and the Ural Mountains, Siberia; it is also found in Moravia, Sweden, and the Isle of Elba. Tourmaline occurs in granite, particularly the albitic varieties, schists, and dolomite. Crystallisation of the tourmaline is rhombohedral, hemimorphic, and the prisms have three, six, nine, or twelve sides. In hardness it is equal to quartz and approaches topaz, being 7 to 7.5. Its lustre is vitreous, it ranges from transparent to opaque, and is doubly refractive to a high degree. Its cleavage is perfect on the basal plane, breaking with uneven fractures. Its specific gravity is from 2.94 to 3.15.
Tourmaline is one of the most dichroic stones, and individual specimens vary more from others in composition and proportion than is the case in almost any other mineral. In colour, black shading to light brown is the commonest; but blue, green, red, and pink are usually desired. Some of the shades are very rich; and richness, rather than brilliancy, is the quality which appeals to the artistic eye of the connoisseur. Curious specimens have shown internal shades of red and external of green, while others differ in colour toward the extremities. Dana distinguishes varieties as follows: rubellite, shades of red, frequently transparent (two of the finest known specimens of this variety are in the British national collection in the Natural History Museum at South Kensington, England); indicolite, indigo blue--Berlin blue, the Brazilian sapphire of jewellers; Brazilian emerald; chrysolite (or peridot), green and transparent; peridot of Ceylon, honey-yellow; achroite, colourless; aphrizite, black; and columnar and black, without cleavage or trace of fibrous texture.
Tourmaline heated, like some other minerals in which one termination differs in form from the other, develops electricity, with the effect of making of the ends positive and negative poles. Sections of tourmaline crystals cut parallel to the axis have the property of polarising light. Tourmaline can be fused under the blowpipe to a spongy enamel; it melts with borax to transparent glass. Tourmaline is cut step and brilliant.
Twin-coloured tourmaline is strongly doubly refractive; green shows yellow and greenish blue; yellowish green, yellow and green; reddish brown, light and dark brown; red, pink and dark red; blue, light and dark blue. The green tends toward blue while the blue has a greenish tendency. Some brown tourmalines have mixed colours.
In considering shades when selecting tourmalines, a medium bright green is better than the lighter or that which appears blackish. The pink should be deep and clean, ruby-like. A rich amber brown is most desirable of the brown shades. Red tourmaline is occasionally so like the ruby that it might deceive any but the expert and his recourse to a scientific test; the hardness of the ruby would of course decide it. In its two-colour character, the tourmaline resembles the ruby but surpasses it; the colour of the tourmaline is not so deep nor is it so lustrous as the ruby's, but it is frequently more transparent. While some red tourmaline resembles spinel, the latter is singly refractive and has a yellow tint. Red topaz is harder and of greater specific gravity than red tourmaline. The two colours of the topaz are red and yellow while the tourmaline's are rose and dark red. Sapphire is harder than tourmaline and clear blue, while tourmaline is greenish blue. Aquamarine is a water blue and is harder than tourmaline, but is of a lower specific gravity. The several other colour varieties of tourmaline bear sometimes a strong resemblance to other stones, but are easily distinguished by the expert, usually without further test than the employment of the dichroiscope. Tourmaline has sometimes been confounded with some of the fine green diopsides found in New York State. |
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Precious Stones Guide Vol 4
>> Properties and Characteristics of Tourmalines
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