braunite

from The Century Dictionary.

• noun A native oxid of manganese, containing also 20 per cent. of manganese silicate.
• noun It occurs in tetragonal crystals of a brownish-black color in Thuringia, the Harz, Piedmont, and elsewhere.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Min.) A native oxide of manganese, of dark brownish black color. It was named from a Mr. Braun of Gotha.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mineralogy A silicate mineral containing divalent and trivalent manganese and forming grey-black tetragonal crystals.