Large crystals in igneous rocks are usually attributed to a slow rate of crystallization. However, with pegmatites, large crystals are attributed to low-viscosity fluids that allow ions to be very mobile. ... This allows them to move about freely and form crystals rapidly. This is why crystals of a pegmatite grow so large.

Does pegmatite have large crystals?

Crystal size is the most striking feature of pegmatites, with crystals usually over 50 mm (2.0 in) in size. Individual crystals over 10 m (33 ft) long have been found, and many of the world's largest crystals were found within pegmatites.

What causes a rock to have large crystals?

When magma cools underground, it cools very slowly and when lava cools above ground, it cools quickly. When magma and lava cool, mineral crystals start to form in the molten rock. Plutonic rocks, which cool slowly underground, have large crystals because the crystals had enough time to grow to a large size.