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Tourmaline is a borosilicate of aluminium, alkali metals, iron, and magnesium, containing water of constitution and a little fluorine. It may be regarded as derived from H9Al3B2(OH)2Si4O19 by the replacement of the nine hydrogen atoms by metals. It is a trigonal mineral (ditrigonal pyramidal; a:c = 1:0.4474); hardness, 7; density, 3.1. Tourmaline is an important rock-forming mineral; it is interesting as an excellent example of a crystal exhibiting polar symmetry and pyro-electric properties, and as a valuable material for the production of plane-polarised light. The colour of tourmaline varies greatly with its chemical composition, from colourless to red, green, brown, and black. Excellent tourmaline crystals, suitable for use as gem-stones, are found in Ceylon and also in Brazil.

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