Reference work entry
Tourmaline is the collective name for the tourmaline family of minerals. It is a crystalline boron silicate mineral with additional elements, such as aluminium, sodium, iron and magnesium. It crystallizes in the trigonal system, and the crystals are prismatic with transverse streaks. It has a rounded triangular cross-section, and it sometimes appears in the form of blocky and dense aggregates. Tourmaline has seven species: elbaite (with many colours), aphrizite (black), buergerite, dravite (brown), rubellite (pink), chromium dravite (green) and calcium dravite (black, brown, yellowish-green). The crystals are usually pink at one end and green at the other, and they can grow to very large sizes. The streak is colourless. It has transparent to translucent diaphaneity, a vitreous lustre, a specific gravity of 3.0–3.2, a Mohs hardness of 7–7.5, and uneven to conchoidal fractures. It mainly forms in granite, pegmatite and some metamorphic rocks. It coexists with many minerals, such as...
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