Pegmatite is a very coarse-grained igneous rock that has a grain size of 20 mm or more; such rocks are referred to as pegmatitic.
Most pegmatites consist of granite that contains quartz, feldspar and mica. Pegmatites are important because they often contain other rare earth minerals and gemstones, such as aquamarine, tourmaline, topaz, fluorite, apatite, tin, and tungsten, among others. Crystals over 10 meters across have been found.
Granitic pegmatites, boron-granitic pegmatites, lithia pegmatites, and boron-lithia pegmatites exist.
In initial Geology classes, students are taught that large crystals are a result of magmas cooling slowly, and that smaller crystals result from faster coolings. Pegmatities are an exception to this rule.
Pegmatites are formed from magma that cools quite quickly, sometimes within a matter of days. Often, what happens is that a magma dike or sill intrudes into much cooler rock without actually breaking through to the surface. For reasons still poorly understood, this rock is able to grow large crystals despite its rapid cooling. This is likely due to the action of water, which is very important in all crystal reactions.
Despite their rapid cooling, pegmatites can have very large crystals, sometimes many feet in length. The effects of water can also concentrate rarer elements in the pegmatites. Consequently, it is not uncommon to find rare minerals and even gem stones in pegmatites. For example, beautiful crystals of aquamarines and topaz can be found in pegmatites in the mountains of Colorado and Idaho. Pegmatites are also a source for rare earth minerals such as columbite and tantalite.
Pegmatites tend to form veins and thick bands in granites. They can also form pockets that can contain beautifully formed crystals. This is because the crystals are free to grow in the empty space of the pocket without crowding or distortion.