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Glittering Stones >> Semi
Precious Stones >> Semi Precious Stone Tourmaline
Tourmaline
The mineral tourmaline is a complex silicate of aluminium and boron,
but because of isomorphous[?] replacements it varies widely in composition
with iron, magnesium and lithium also entering into the combination to
a greater or lesser extent. Tourmaline belongs to the hexagonal system,
with long, slender, prismatic acicular crystals that are usually terminated
with three faces of a rhombohedron[?]. The smaller crystals are often
found in a radial pattern, and collumnar masses are common. The prisms
are usually three, six, or nine sided with heavy vertical striations producing
a rounded effect. Tourmaline is essentially without cleavage, its fracture
is conchoidal to even, its hardness is 7-7.5, its specific gravity is
2.9-3.2, and it has a vitreous lustre inclining to resinous.
Tourmaline has a wide variety of colors. Common tourmaline can be black,
bluish-black, brown, blue, green, red or pink; transparent varieties can
be colourless (rare), various shades of rose or pink, greens, blues and
brown. Bi-colored crystals are common and can be green at one end and
pink at the other, or green on the outside and pink within, which is very
attractive in the case of transparent tourmalines.
The opaque black tourmalines were originally called schorl, a term which
was applied to all tourmalines until 1703 when the word tourmaline was
introduced as a corruption of the Ceylonese[?] word turamali. |
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