There are many named varieties. Usually, only the color determines the variety, but sometimes the composition as well. The main groups of gemstone quality are Schorl (Fe) - black, Dravite (Mg) - brown, Elbaite (Li) which has the following varieties: Achroite - colorless, Rubellite - pink, red, reddish-violet, Verdelite - green, Indicolite - blue, Siberite - violet, and Tsilaisite (Mn) - yellowish-brown, and Liddicoatite (Ca) - multicolored. Named after the locality in Brazil is Paraiba tourmaline, which has a very intense color partly created by a small amount of copper. Chrome tourmaline gets its color from chromium and more or less vanadium.
|Hardness:||7 – 7½|
|Double refraction:||0.02 (medium)|
|Pleochroism:||Strong (sometimes very strong)|
Facet angles (pavilion/crown):
All grit sizes
|Vargas:||Aluminum oxide on tin/lead|
|Olson:||Aluminum oxide on tin|
|Soukup:||Aluminum or cerium oxide on tin/lead|
|Christiansen:||Aluminum oxide on tin/lead, tin oxide on tin|
|Perkins:||Diamond on BATT or Corian|
|MDR:||Aluminum oxide on tin, tin or aluminum oxide on typemetal|
|Raytech:||Aluminum oxide on Fast Lap, tin or phenolic|
|Christiansen:||Aluminum, tin, or chromium oxide on leather or wood, diamond 50k/100k|
|Cox:||Aluminum or chromium oxide on leather|
|Covington:||Aluminum or chromium oxide on leather, diamond on canvas|
According to the best color (unfortunately often in the C-axis, the short end). Sometimes the C-axis is "closed," meaning very dark, and a cut that allows little light transmission in that direction can make a stone that is less dark on the sides.
Many different variations of irradiation and/or heat treatments are used.
Long, narrow crystals are often more brittle and have more inclusions than round, grain-shaped pieces. Heat them gently or use cold dop.
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