A complex borosilicate of aluminium and alkali, with Iron, magnesium, and other cations. The name is apparently derived from the Sinhalese turamali, referring to gems of unknown identity—probably zircons.
Crystal system Trigonal.
Appearance It usually occurs as long, three-sided prisms, which are often well terminated; but sometimes it is found as parallel or radiating groups of long, thin striated prisms. It has one of the widest color ranges in the mineral world. The most common color is black, but tourmaline may be pink, violet-red, brownish yellow, blackish brown, various shades of green, light blue, blue-green, dark blue, and (rarely) colorless. Fine crystals with concentric zoning are found; typically they are red on the inside and green on the outside, and are known as “watermelon” tourmalines Also found are crystals with transverse zoning, the color of the crystal gradually changing from one end to the other, as in the “Moor's head" crystals found on the island of Elba. Tourmaline has good resistance to weathering and is therefore often found in alluvium.
Physical properties It has a hardness of 7 and a density of approximately 3.02 to 3.20 g/ems, with some variation between types; from 3.03-3.06 g/cma for the pink, red, brown, and light green varieties; 3.08 g/cm3 for the dark green; 3.10 g/cm?’ for the dark blue to yellow; and 3.15-3.20 g/cma for the black. The refractive indices are about ne 1.62, no 1.64, thus with quite marked birefringence, and strong pleochroism.
Genesis Tourmaline is found in differentiated dikes of silica-rich intrusive rocks and is quite common around grantite, where pegmatitic, pneumatolytic mineralizations are
Occurrence It is widely distributed, the most common variety being black tourmaline, which is of no value an a gem. The most famous deposits are in Sri Lanka, the Soviet Union (Urals), Afghanistan, Burma, the United States (California, Maine, Connecticut), Brazil, Tanzania, Zimbabwe,and Namibia.