The Collaborative International Dictionary of English v.0.48:

Quartzite \Quartz"ite\, n. [Cf. F. quartzite.] (Min.)
   Massive quartz occurring as a rock; a metamorphosed
   sandstone; -- called also quartz rock.
   [1913 Webster]

WordNet (r) 3.0 (2006):

quartzite
    n 1: hard metamorphic rock consisting essentially of
         interlocking quartz crystals