Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

tridymite
     n : a mineral form of silica