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Tourmaline belongs to the trigonal crystal system and occurs as long, slender to
thick prismatic and columnar crystals that are usually triangular in cross-section.
All hemimorphic crystals are piezoelectric, and are often pyroelectric as well.
Tourmaline crystals when warmed become positively charged at one end and
Tourmaline has a wide variety of colors. Usually, iron-rich tourmalines are black
to bluish-black to deep brown, while magnesium-rich varieties are brown to
yellow, and lithium-rich tourmalines are practically any color: blue, green, red,
yellow, pink etc. Rarely, it is colorless. Bi-colored and multicolored crystals are
relatively common, reflecting variations of fluid chemistry during crystallization.
Crystals may be green at one end and pink at the other, or green on the outside
and pink inside: this type is called watermelon tourmaline. Some forms of
tourmaline are dichroic, in that they appear to change color when viewed from
different directions.
Tourmaline is used in jewelry, pressure gauges, and specialist microphones. In
jewelry, blue indicolite is the most expensive, followed by green verdelite and pink
rubellite. Ironically the rarest variety, colorless achroite, is not appreciated and is
the least expensive of the transparent tourmalines.
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