Tourmaline

Tourmaline is a complex silicate of boron with aluminium and several other metals. It usually occurs in well-developed hexagonal crystals. Hardness of tourmaline varies from 7 to 7.5 and the specific gravity ranges from 2.9 to 3.2. It is usually black in colour and its streak is uncoloured. Attractive coloured and transparent tourmalines are used as gem stones. Light green tourmaline of non-gem grade is reported to occur near Kiranur in Tiruchirapalli district of Tamilnadu and mica mine inNellore district.