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tourmaline: A group of minerals composed of complex cyclosilicates containing boron with the general formula NaR₃ ²⁺Al₆B₃Si₆O₂₇(H,F)4, where R = Fe²⁺, Mg, or (Al + Li). The crystals are trigonal, elongated, and variably coloured, the two ends of the crystals often having different colours. Tourmaline is used as a gemstone and because of its double refraction and piezoelectric properties is also used in polarizers and some pressure gauges.

Tourmaline, which is a borosilicate, is the main source of boron found in soil. It is a mixture of silicates and borates from where boron is extracted. It is available as a complex cyclosilicate, Al₆Y₃Na[Si₆O₁₈](BO₃)₃(OHF)₄, with Y = Mg, Fe, Mn or Li. The rhombohedral system has the shape of an elongated rod or needle type prism. Tourmaline is used as a gemstone, and because of its double refraction and piezo-electric properties, it is used in polarizers and in some pressure gauges. The best known variety of tourmaline is black tourmaline which is rich in iron and is a common mineral in igneous and metamorphic rocks.
Magnesium-rich tourmaline is called dravite. Lithium-rich varieties are called elbaite. Ferruginous varieties are called schorlite which is insoluble in water and resistant to weathering and hence releases boron slowly.