The Collaborative International Dictionary of English v.0.48:

Tetradymite \Tet*rad"y*mite\, n. [Gr. tetra`dymos fourfold. So
   named from its occurrence in compound twin crystals, or
   fourlings.] (Min.)
   A telluride of bismuth. It is of a pale steel-gray color and
   metallic luster, and usually occurs in foliated masses.
   Called also telluric bismuth.
   [1913 Webster]