crystallite

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of numerous minute rudimentary, crystalline bodies of unknown composition found in glassy igneous rocks.

Century Dictionary and Cyclopedia

1. n. Whinstone cooled slowly after fusion.
2. n. The term suggested by Vogelsang as a general name for aggregations of globulites in various forms. See cumulite, margarite, and longulite. These terms are used exclusively in describing various groupings of minute drop-like bodies (globulites), seen under the microscope in thin sections of rocks. See globulite.

Wiktionary

1. n. A small region of a solid that consists of a single crystal; a grain

GNU Webster's 1913

1. n. 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.

WordNet 3.0

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