Pegmatite is an exceptionally coarse-grained igneous rock, typically composed of quartz, feldspar, and mica. These rocks form during the final stages of magma crystallization and often contain rare minerals, making them important for studying the late-stage processes of magma evolution and for the extraction of rare elements like lithium and tantalum.
Reference: London, D. (2008). “Pegmatites.” Canadian Mineralogist Special Publication 10.