Tourmaline

Tourmaline
Schorl 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
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) H-M symbol: (3m)
Identification
Color	Most commonly black, but can range from colorless, brown, violet, yellow, orange, blue, red, green, pink, or bi-colored, or even tri-colored.
Crystal habit	Parallel and elongated. Acicular prisms, sometimes radiating. Massive. Scattered grains (in granite).
Cleavage	Indistinct
Fracture	Uneven, small conchoidal, brittle
Mohs scale hardness	7–7.5
Luster	Vitreous, sometimes resinous
Streak	White
Specific gravity	3.06 (+.20 -.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 .020 but in dark stones it may reach .040
Pleochroism	typically moderate to strong Red Tourmaline: Definite; dark red,light red Green Tourmaline: Strong; dark green, yellow-green Brown Tourmaline: Definite; dark brown, light brown Blue Tourmaline: Strong; dark blue, light blue
Dispersion	.017
Ultraviolet fluorescence	pink stones—inert to very weak red to violet in long and short wave
Absorption spectra	a strong narrow band at 498 nm, and almost complete absorption of red down to 640nm in blue and green stones; red and pink stones show lines at 458 and 451nm as well as a broad band in the green spectrum

Tourmaline (pronunciation: /ˈtʊərməliːn/ TOOR-mə-leen) is a crystalline boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gemstone comes in a wide variety of colors. The name comes from the Tamil and Sinhalese word "Turmali" (තුරමලි) or "Thoramalli" (තෝරමල්ලි), which applied to different gemstones found in Sri Lanka.

Brightly colored Sri Lankan gem tourmalines were brought to Europe in great quantities by the Dutch East India Company to satisfy a demand for curiosities and gems. At the time it was not realised that schorl and tourmaline were the same mineral (it was only about 1703 that it was discovered that some colored gems weren't zircons). Tourmaline was sometimes called the "Ceylonese [Sri Lankan] Magnet" because it could attract and then repel hot ashes due to its pyroelectric properties.