The Collaborative International Dictionary of English v.0.48:

Crystallite \Crys"tal*lite\ (kr?s"tal-l?t), n. [See Crystal.]
   (Min.)
   A minute mineral form like those common in glassy volcanic
   rocks and some slags, not having a definite crystalline
   outline and not referable to any mineral species, but marking
   the first step in the crystallization process. According to
   their form crystallites are called trichites, belonites,
   globulites, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

crystallite
    n 1: any of numerous minute rudimentary crystalline bodies of
         unknown composition found in glassy igneous rock