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==General description==
Pegmatites are exceptionally coarse-grained [[igneous rock]]s<ref name=Jackson1997>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=pegmatite}}</ref> composed of interlocking [[crystal]]s, with individual crystals usually over {{convert|1|cm|sigfig=1|sp=us}} in size and sometimes exceeding {{convert|1|m|ft|sigfig=1|sp=us}}.<ref name=BlattTracy1980>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0716724383 |edition=2nd |page=73}}</ref> Most pegmatites have a composition similar to [[granite]], so that their most common minerals are [[quartz]], [[feldspar]], and [[mica]].<ref name=BlattTracy1980/><ref name=KleinHurlbut1993>{{cite book |last1=Klein |first1=Cornelis |last2=Hurlbut | first2=Cornelius S. Jr. |title=Manual of mineralogy : (after James D. Dana) |date=1993 |publisher=Wiley |location=New York |isbn=047157452X |edition=21st |page=568}}</ref> However, other pegmatite compositions are known, including compositions similar to [[nepheline syenite]]<ref name=KleinHurlbut1993/> or [[gabbro]].<ref name=BlattTracy1980/> The term ''pegmatite'' is thus purely a [[Texture (geology)|textural]] description.<ref name=PhilpottsAgue2009_255>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |page=255}}</ref><ref name="bgs">{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=20–21}}</ref> Geologists typically prefix the term with a compositional description, so that ''granitic pegmatite'' is a pegmatite with the composition of granite while ''nepheline syenite pegmatite'' is a pegmatite with the composition of nepheline syenite.<ref name=PhilpottsAgue2009_255/> However, the [[British Geological Survey]] (BGS) discourages this usage, preferring terms like ''biotite-quartz-feldspar pegmatite'' for a pegmatite with a typical granitic composition, dominated by feldspar with lesser quartz and biotite. Under BGS terminology, a ''pegmatitic rock'' (for example, a ''pegmatitic gabbro'') is a coarse-grained rock containing patches of much coarser-grained rock of essentially the same composition.<ref name="bgs"/>
Individual crystals in pegmatites can be enormous in size. It is likely that the largest crystals ever found were feldspar crystals in pegmatites from [[Karelia]] with masses of thousands of tons. Quartz crystals with masses measured in thousands of pounds<ref name=KleinHurlbut1993/> and micas over {{convert|10|m|ft||sp=us}} across and {{convert|4|m|ft||sp=us}} thick have been found.<ref name="McBirney1984">{{cite book |last1=McBirney |first1=Alexander R. |title=Igneous petrology |date=1984 |publisher=Freeman, Cooper |location=San Francisco, Calif. |isbn=0198578105 |pages=349–350}}</ref> [[Spodumene]] crystals over {{convert|40|ft|m|order=flip|sp=us}} long have been found in the [[Black Hills]] of [[South Dakota]], and [[beryl]] crystals {{convert|27|ft|m|order=flip|sp=us}} long and {{convert|6|ft|m|order=flip|sp=us}} in diameter have been found at [[Albany, Maine]].<ref name=KleinHurlbut1993/> The largest beryl crystal ever found was from Malakialina on Madagascar, weighing about 380 tons, with a length of {{cvt|18|m|ft}} and a crosscut of {{cvt|3.5|m|ft}}.<ref>{{cite web |url=http://minsocam.org/MSA/collectors_corner/arc/large_crystals.htm |title=The largest crystals by Peter C. Rickwood |publisher=American Mineralogist}}</ref>
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