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The word tourmaline comes from the Singhalese word "turamali" meaning mixed-colored stones as they were often confused with other stones. The tourmaline gemsstone group consisting of nine distinct mineral species all having the same mineral structure but varying in chemical composition, color and properties. The chemical formulas of the nine species are as follows: Dravite: NaMg3Al6B3Si6O27(OH)3(OH,F) Uvite: CAMg3(AlMg)B3SiO6O27(OH)3(OH,F) Schorl:Na(Fe,Mn)3Al6B3SiO27(OH)3(OH,F) Elbaite: Na(Li,Al)3Al6B3Si6O27(OH)3(OH,F) Liddicoatite:Ca(Li,Al)3Al6B3Si6O27(OH)3(OH,F) Buergerite:NaFe3Al6B3Si6O27(OH)3(OH,F) Chromdravite:NaMg3Cr6(BO3)3Si6O18(OH)4 Tsilaisite:NA(Mn,Al)3Al6(BO3)3Si6O18(O,OH,F)4 Ferridravite:(Na.K)(Mg,FE+2)3Fe6+3(BO3)3Si6O18(O,OH)4 Crystal habbit is Hexagonal (trigonal), crystal shape is elongated with terminations and parallel striations along the length of the crysta.
Tourmalines comes in all colors of the spectrum. Bi-color, tri-color, parti-color, concentric colors are all found in tourmalines. Indicolites of predominant blue shades and pink and red hues of rubellites are prized and command the highest prices. Cat's eye tourmaline usually in green shades displaying sharp chatoyancy are also much appreciated. Deep chrome colored tourmalines which are not too dark are rare and sought after by collectors and gem dealers alike. Liddicoatite was named after the famed president of the Gemological Institute of America, Richard T Liddicoat and is a relatively new gem in the trade. Dravite, uvite and schorl are dark brown/green or black. uvite from Sri Lanka are yellow and brown/green colors. Achroite is colorless. Buergerite is dark brown/black and sometimes know to contain inclusions displaying bronze-colored iridescence or schiller under the surface.
Chromdravite is an instense dark green from Russia. Other sources include California, Maine,Burma, Tanzania, Zambia, Kenya, Mozanbique, Madagascar and Brazil. Tourmaline occurs is very large crystals but are often flawed and cracked. Clean stones therefore are rare and has a market premium when available. Hardness is 7 to 7.5 on the Moh's scale. Luster is vitreous. Birefringence ranges from 0.016 to 0.028 depending on the variety. Pleochroism is strong. The diversity of colors and subtle brilliance due to strong double refractive properties gives the gem a unique appearance different from any gemstone. |
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