'Crystallite' definitions:

Definition of 'crystallite'

noun

Any of numerous minute rudimentary crystalline bodies of unknown composition found in glassy igneous rock

From: GCIDE

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]