Pegmatite

Physical Properties

Hardness: 6-7 (Mohs scale); Color: White, cream, and grey with silvery flashes; Luster: Vitreous to pearly; Crystal structure: Coarse interlocking crystals; Cleavage: Good in feldspar/mica components; Specific gravity: 2.5-2.8

Formation & Geological History

Formed during the final stage of a magma's crystallization, usually in hydrothermal veins or at the margins of large plutons where high water content allows for very rapid crystal growth. These can date from the Precambrian to more recent Cenozoic eras.

Uses & Applications

Primary source of rare earth elements, lithium, beryl, and industrial minerals like mica and feldspar. High-quality specimens are used in jewelry and lapidary arts.

Geological Facts

Pegmatites are famous for containing the world's largest crystals, sometimes exceeding 10 meters in length. They are essentially 'geological garbage cans' because they concentrate rare elements that don't fit into the crystal structures of common rock-forming minerals.

Field Identification & Locations

Identify in the field by exceptionally large crystal sizes (over 1cm) relative to the surrounding rock. Common in mountain belts like the Appalachians or the Black Hills. Look for the characteristic 'sheen' of mica and 'blocky' cleavage of feldspar.

Identified on: 5/15/2026

Mode: Standard