knebelite

from The Century Dictionary.

• noun A mineral of a gray, dirty-white, brownish-green, or green color, a silicate of iron and manganese, belonging to the chrysolite group, found at Ilmenau in Thuringia and at Dannemora in Sweden.

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

• noun (Min.) A mineral of a gray, red, brown, or green color, and glistening luster. It is a silicate of iron and manganese.

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

• noun mineralogy A silicate mineral, a manganese variety of fayalite that forms dark green orthorhombic crystals.