The Collaborative International Dictionary of English v.0.48:

Tridymite \Trid"y*mite\, n. [Gr. ? threefold. So called in
   allusion to the supposed threefold nature of the twin
   crystals.] (Min.)
   Pure silica, like quartz, but crystallizing in hexagonal
   tables. It is found in trachyte and similar rocks.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tridymite
    n 1: a mineral form of silica