tour·ma·line /ˈtʊrmələn, ˌlin/
Tour·ma·line n. Min. A mineral occurring 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. [Written also turmaline .]
Note: ☞ Crystals of tourmaline when heated exhibit electric polarity (see Pyroelectric, n.). Tourmaline is also used in the form of a polariscope called tourmaline tongs.
n : a mineral that is a complex borosilicate and hydroxide of
aluminum containing iron and magnesium and calcium and
lithium and sodium; it is usually black but occurs in
transparent colored forms that are used as gemstones