Tourmaline
Tourmaline (/ˈtʊərməlɪn, -ˌliːn/ TOOR-mə-lin, -leen) is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. This gemstone comes in a wide variety of colors.
| Tourmaline | |
|---|---|
| General | |
| Category | Cyclosilicate |
| Formula (repeating unit) | (Ca,K,Na, ▢)(Al,Fe,Li,Mg,Mn)3(Al,Cr,Fe,V)6 (BO3)3(Si,Al,B)6O18(OH,F)4 |
| IMA symbol | Tur |
| Crystal system | Trigonal |
| Crystal class | Ditrigonal pyramidal (3m) H-M symbol: (3m) |
| Space group | R3m (no. 160) |
| Identification | |
| Color | Most commonly black, but can range from colorless to brown, red, orange, yellow, green, blue, violet, pink, or hues in-between. It can also be bi-colored, or even tri-colored. Rarely, it can be found as neon green or electric blue. |
| Crystal habit | Parallel and elongated; acicular prisms, sometimes radiating; massive; scattered grains (in granite) |
| Cleavage | Indistinct |
| Fracture | Uneven, small conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 7.0–7.5 |
| Luster | Vitreous, sometimes resinous |
| Streak | White |
| Diaphaneity | Translucent to opaque |
| Specific gravity | 3.06+0.20–0.06 |
| Density | 2.82–3.32 |
| Polish luster | Vitreous |
| Optical properties | Double-refractive, uniaxial negative |
| Refractive index | nω = 1.635–1.675 nε = 1.610–1.650 |
| Birefringence | −0.018 to −0.040; typically about −0.020 but in dark stones it may reach −0.040 |
| Pleochroism |
|
| Dispersion | 0.017 |
| Ultraviolet fluorescence | Pink stones; inert to very weak red to violet in long and short wave |
| Absorption spectra | Strong narrow band at 498 nm, and almost complete absorption of red down to 640 nm in blue and green stones; red and pink stones show lines at 458 and 451 nm, as well as a broad band in the green spectrum |
The name is derived from the Sinhalese tōramalli (ටෝරමල්ලි), which refers to the carnelian gemstones.
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