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Chemical formula: Al2SiO4(F9OH)2 + Cr
Hardness: 8
Cleavage: Perfect basal ( one direction). Fracture-conchoidal. Brittle.
Luster: vitreous
Crystal habbit: Orthorhombic. Prismatic, well formed, water worn pebbles, stimpy, very large, granular
Density: 3.57-3.57
Occurs in dark yellow, light yellow, brown, pale blue, dark rose red, pink, lilac, brown, yellow brown, gray brown, green, light green, colorless
Sources: Nigeria, Sri Lanka, Burma, Brazil, Russia, Pakistan, Utah, Australia, Namibia, Zimbabwe, China
Most of the blue topaz seen in jewelry nowadays have been treated with cobalt 60, electrons or neutron atomic particles to create a blue color. Sky blue being a lighter shade resembling a "sky blue" at noon on a clear day, Swiss blue is more intense almost neon like, while London blue has a bluish darker inkish cast. Prices of irradiated blue topaz has come down to a few dollar and cents per carat due to over production. You can own a piece for a few to tens of dollars for a very nice piece.
Precious topaz on the other hand still a rare item especially the pinkish, red, sherry and peach colors; these are orginally orange crystal containing Cr and after heating turns to a pink hue and show a Cr spectrum.
Sources include Minais Gerais, Brazil and Mardan, Pakistan.
Burma, Sri Lanka, China also produce topaz. Treatable rough coming in large quantities by the hundred of kilos mostly from Sri Lanka, China, Nigeria and Brazil.
Natural colors of red,pink and orange are exceeding rare.
Thermoluminescence may determine treated versus natural colors howver not enough gemological study has been reported on this subject.
Topaz is vey large sizes of over 20,000 carats have been cut and can be viewed in the Natural History Museum of Los Angeles and New York as well as the Smithsonian Nuseum in Washington DC.
Sherry, champagne, light green and blue are desirable colors for any gem collection.
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