pyrolusite
    n 1: a mineral consisting of manganese dioxide; an important
         source of manganese

Pyrolusite \Pyr`o*lu"site\, n. [Pyro- + Gr. ? to loose, or ? a
   loosing.] (Min.)
   Manganese dioxide, a mineral of an iron-black or dark
   steel-gray color and metallic luster, usually soft.
   Pyrolusite parts with its oxygen at a red heat, and is
   extensively used in discharging the brown and green tints of
   glass (whence its name).
   [1913 Webster]