Tourmaline

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[1][2]
IMA symbol	Tur[3]
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[1]
Density	2.82–3.32
Polish luster	Vitreous[1]
Optical properties	Double-refractive, uniaxial negative[1]
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[1]
Pleochroism	Typically moderate to strong[1] Red: definite; dark red, light red Green: strong; dark green, yellow-green Brown: definite; dark brown, light brown Blue: strong; dark blue, light blue
Dispersion	0.017[1]
Ultraviolet fluorescence	Pink stones; inert to very weak red to violet in long and short wave[1]
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[1]

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.

The name is derived from the Sinhalese tōramalli (ටෝරමල්ලි), which refers to the carnelian gemstones.^[4]