Tourmaline is the most commonly found in granite pegmatites. It is usually black in colour, but lighter coloured gem varieties are also found. It occurs usually in three-sided or six-sided prisms terminated by rhombohedral or scalenohedral planes. Black tourmaline (schorl) is the most common variety, but there are also other varieties, as the blue (indicolite), red (rubellite), also green, brown, and white. The red and green varieties when transparent are valued as jewels. Crystals of tourmaline when heated exhibit electric polarity. Tourmaline is also used in the form of a polariscope called tourmaline tongs. Tourmaline has the formulae (Na, Ca)(Al,Fe,Li,Mg)3A16(BO3)3(Si6O18) and a relative hardness of 8.
No related terms.