A pegmatite is an exceptionally coarse-grained intrusive igneous rock with large interlocking crystals (usually > one inch), typically forming as masses in dikes and veins, especially along the margins of batholiths. A granitic pegmatite has the mineralogy of a granite and abnormally large grains, whereas a gabbroic pegmatite has the mineralogy of a gabbro and very large grains. Pegmatites represent the last and most hydrous magma to crystallize, and sometimes contain rare minerals enriched in the elements boron, lithium, uranium, and rare earths.
-- Excerpts from:
Margaret O. Plank and William S. Schenck, 1998; Delaware Piedmont Geology: Delaware Geological Survey, University of Delaware, Delaware Geological Survey Website, 2001; British Columbia Ministry of Energy and Mines Website, 2002, and Philip Stoffer, 2002, Rocks and Geology in the San Francisco Bay Region, USGS Bulletin 2195