A rock containing giant crystals. Pegmatites form after crystallization is nearly complete in a subduction zone. The last species to crystallize become concentrated in small water-rich pockets or veins. These include H₂O, CO₂, Nb, Ta, B, Li, and Be. Under the large overburden pressure, the pockets cool extremely slowly, forming huge crystals in the vapor pockets. A vein containing a large number of these pockets is termed a pegmatite.