Species and varieties
Tourmaline is a group of complex borosilicate minerals sharing a common trigonal crystal structure. The gem-grade tourmalines are predominantly elbaites — the lithium-bearing series — which span the widest commercial color range. Other tourmaline species include schorl (black, ferrous), dravite (brown, magnesium-bearing), uvite, and liddicoatite.
Mohs hardness is 7 to 7.5; specific gravity is 2.9 to 3.2. Tourmaline is uniaxial and negative.
Color terminology
Trade color terms in current use:
- Indicolite — blue elbaite.
- Rubellite — pink to red elbaite, color stable in incandescent light.
- Verdelite — green elbaite (less common term).
- Watermelon — bicolored crystal with pink core and green rim, cut perpendicular to the c-axis.
- Paraíba — copper-bearing elbaite, electric blue-green; first found at the Paraíba State, Brazil, deposits in the late 1980s and now found in Mozambique and Nigeria as well.
- Chrome tourmaline — chromium-bearing dravite or elbaite, intense green.
Sources
- Brazil — Minas Gerais and Paraíba/Rio Grande do Norte; the original Paraíba locality and historic source of indicolite, rubellite, and bicolor material.
- Nigeria — significant source of cuprian (Paraíba-type) and standard elbaite since the 2000s.
- Mozambique — major contemporary source of cuprian tourmaline; in commercial production since approximately 2001.
- Afghanistan — Nuristan and Kunar provinces produce elbaite in saturated colors.
- Madagascar — multi-color elbaite, often with strong pleochroism.
- California, USA — historic sources at Pala and Mesa Grande.
Treatments
- Heat treatment is widely applied to tourmaline, particularly to lighten over-saturated rubellite and to improve color in pink and red material. Treatment is generally disclosed.
- Irradiation is used to deepen pink to red colors; treatment is disclosed in some markets.
- Cuprian Paraíba-type tourmaline is occasionally heat-treated; the practice is disclosed.
Lapidary considerations
Tourmaline is strongly dichroic — the c-axis "closed" orientation typically produces deep, saturated color while the c-axis "open" orientation can produce a much lighter or different hue. Orientation during preform layout is consequential for the finished stone's appearance.
Tourmaline cuts cleanly with diamond grits. Final polish is conventionally performed with tin oxide or cerium oxide; some cutters report better results on rubellite with diamond paste at 100,000 mesh.