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Tourmaline

Tourmaline

The tourmaline group refers to a number of related species and varieties. Colour: Colourless, pink, red, yellow, brown, green, blue, violet, black, multicoloured Colour of streak: White Mohs' hardness: 7-71/2 Density: 2.82-3.32 Cleavage: Indistinct Fracture: Uneven, small Conchoidal, brittle Crystal system: (Trigonal), long crystals with triangular cross section and rounded sides, definite striation parallel to main axis Transparency: Transparent to opaque Refractive index: 1.614-1.666 Double refraction: -0.014 to -0.032 Dispersion: 0.017 (0.009-0.011) Pleochroism: Red t: definite: dark red, light red; Brown t: definite: dark brown, light brown; Green t: strong: dark green, yellow-green; Blue t: strong: dark blue, light blue Absorption spectrum: Often extremely faint Fluorescence: Weak or none Even though tourmaline has been known since antiquity in the Mediterranean region, the Dutch imported it only in 1703 from Sri Lanka to Western and Central Europe. They gave the new gems a Sinhalese name, Turamali, which is thought to mean "stone with mixed colours." According to color, the following varieties are recognized in the trade: Achroite (Greek—without colour) colourless or almost colorless, quite rare. Dravite yellow-brown to dark brown, sometimes used for stones not of the dravite species. Indicolite (indigolite) named (after color) blue in all shades. Rubellite (Latin—reddish) pink to red, sometimes with a violet tint; ruby color is the most valuable. Schorl black, very common; used for mourning jewellery. Name derived from an old mining term, sometimes applied to tourmaline that is not actually schorl. Siberite (after finds in Urals) lilac to violet blue. Verdelite ("green stone") green in all shades. Recently, instead of variety names, more and more frequently color names are simply added to the word tourmaline, e.g., yellow tourmaline, pink tourmaline. Mineralogy distinguishes tourmalines according to their chemical composition. The individual members include the following: Buergerite (after U.S. scholar) NaFe3Al6(BO3)3Si6O18(O,F)4 = iron tourmaline Dravite (after a deposit near the river Drave, Carinthia/Austria) NaMg3Al6 (BO3)3Si6O18(OH)4 = magnesium tourmaline Elbaite (after the island of Elba/Italy) Na(Li,Al)3Al6(BO3)3Si6O18(OH)4 = lithium tourmaline Liddicoatite (after U.S. gemologist) Ca(Li,Al)3Al6(BO3)3Si6O18(O,OH,F)4 = calcium tourmaline Schorl (after an old mining expression for "false ore") NaFe3Alfi(BO3)3Si6O18(OH)4 = iron tourmaline Tsilaisite (after a local name in Madagascar) NaMn3Al6(BO3)3Si6O18(OH)4 = manganese tourmaline Uvite (after a province in Sri Lanka) (Ca,Na)(Mg,Fe)3Al5Mg(BO3)3Si5O18(OH,F)4 = magnesium tourmaline
 

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