Tourmaline
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.
Physical properties:
| Chemistry: | Na(Li,Al)3Al6Si6O18(BO3)3(OH)4 |
| Hardness: | 7 – 7½ |
| SG: | 2.98–3.20 |
| Cleavage: | None |
| Fracture: | Low |
| Thermal sensitivity: | Some |
Optical properties:
| RI: | 1.62–1.64 |
| Dispersion: | 0.017 (medium) |
| Double refraction: | 0.02 (medium) |
| Pleochroism: | Strong (sometimes very strong) |
| Critical angle: | 38° |
Facet angles (pavilion/crown):
| Vargas: | 42°/41° |
| Olson: | 40°/43° |
| Soukup: | 43°/39° |
| Roth: | 43°/39° |
| MDR: | 43°/39° |
| Schlagel: | 40°/36° |
| Hashnu: | 42°/41° |
| Sinkankas: | 40°/40° |
| GIA: | 39°/43° |
| Perkins: | 39°/43° |
| Raytech: | 40°/39° |
| Cornwall: | 42°/35° |
| Weikoff: | 42°/39° |
| Carroll: | 42.6°/35.3° |
Cutting lap:
All grit sizes
Facet polishing:
| 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 |
Cabochon polishing:
| 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 |
Orientation:
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.
Treatments/synthetics:
Many different variations of irradiation and/or heat treatments are used.
Tips!
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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