The Collaborative International Dictionary of English v.0.48:

Trachyte \Tra"chyte\, n. [Gr. ? rough, rugged: cg. F. trachyte.]
   (Geol.)
   An igneous rock, usually light gray in color and breaking
   with a rough surface. It consists chiefly of orthoclase
   feldspar with sometimes hornblende and mica.
   [1913 Webster]