axinite

from The Century Dictionary.

• noun A mineral occurring commonly in crystals, whose general form is that of a very oblique rhomb, so flattened that some of its edges become thin and sharp like the edge of an ax (whence its name), also sometimes found in lamellar masses.

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

• noun (Min.) A borosilicate of alumina, iron, and lime, commonly found in glassy, brown crystals with acute edges.

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

• noun mineralogy A borosilicate of aluminum, iron, and lime, commonly found in glassy, brown crystals with acute edges.