Tourmaline is a group of closely related mineral species with the same crystal structure. It has one of the widest color ranges of any gem.
Properties
| Refractive index | 1.624 to 1.644 (GIA, usual); elbaite 1.603 to 1.655 (IGS) |
|---|---|
| Birefringence | 0.018 to 0.040 (GIA); usually about 0.018 to 0.020 (IGS 'usually around 0.020'; LibreTexts 'usually 0.019') |
| Optic character | Doubly refractive (uniaxial) |
| Dispersion | 0.017 |
| Specific gravity | 3.06 (+0.20, -0.06) |
| Hardness (Mohs) | 7 to 7.5 |
| Crystal system | Trigonal |
| Pleochroism | Strong: darker shade of body color (along the c-axis), lighter shade of body color |
| Absorption spectrum | Strong narrow band in blue and green stones, 498 nm; line in red and pink stones, with a broad band in the green, 458 nm; line in red and pink stones, 451 nm |
Published values. They change a little from stone to stone. Our tools do not model tourmaline yet. Sources: GIA · International Gem Society · LibreTexts Gemology · Gemdat
In more detail
Iron, and possibly titanium, give green and blue colors; manganese gives reds and pinks. Trade names include rubellite for pink to red, indicolite for blue, and watermelon for pink inside green. Tourmaline is strongly pleochroic, so cutters orient the table to lighten or darken the face-up color. Heating and irradiation are the main treatments; avoid heat, steam and ultrasonic cleaners.
Related terms
- Paraíba tourmaline: Paraíba tourmaline is a blue, bluish green to greenish blue, green or yellowish green tourmaline. Copper and manganese mainly cause its color, whatever its geographic origin.
- Pleochroism: Pleochroism is the property of a colored doubly refractive stone to show different colors in different directions. It happens because the stone absorbs its rays differently.
- Heat treatment: Heat treatment is controlled heating of a gem to change its color or clarity.
- Irradiation: Irradiation is exposing a gem to artificial radiation to change its color. Heating sometimes follows, to modify the color further.
- Gem variety: A variety is a subdivision of a gem species, usually by color. Ruby and sapphire are varieties of corundum, and emerald is a variety of beryl.