tourmaline

tourmaline
    n 1: 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
    

Tourmaline \Tour"ma*line\, n. [F. tourmaline, cf. It. turmalina,
   tormalina, NL. turmalina, turmalinus; all fr. tournamal, a
   name given to this stone in Ceylon.] (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} .]
   [1913 Webster]

   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.
         [1913 Webster]