Pegmatite

A pegmatite is an igneous rock with a very rough texture, which has large interlocking crystals^[1] that are usually bigger than 1 centimeter (0.4 in) and sometimes bigger than 1 meter (3 ft).^[2]

Pegmatite with blue corundum crystals

Many of the world's largest crystals are found in pegmatites. These include crystals of quartz, mica, beryl, and tourmaline. Some of these crystals are over 10 m (33 ft) long.^[3]

Etymology

The word pegmatite comes from the Homeric Greek word, πήγνυμι (pēgnymi), which means “to bind together." This refers to the interlocked crystals in the rock's texture.^[4]