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Very coarse-grained, igneous rock, usually of granitic composition, in which the individual crystals are at least 2.5 cm long. The crystals are often more than 1 m in length, and may be far larger. Crystallization occurs at a late stage, when the magma is enriched in volatiles and trace elements. Pegmatites may concentrate some rare elements (lithium, boron, fluorine, tantalum, niobium, rare earth elements, and uranium) to economic proportions. — A Dictionary of Geology and Earth Sciences (4 ed.), Oxford University Press