Source : Webster's Revised Unabridged Dictionary (1913)

Quartzite \Quartz"ite\, n. [Cf. F. quartzite.] (Min.)
   Massive quartz occurring as a rock; a metamorphosed
   sandstone; -- called also {quartz rock}.

Source : WordNet®

quartzite
     n : hard metamorphic rock consisting essentially of interlocking
         quartz crystals