Crystal-filled cavities, or pockets, occasionally are found in pegmatite rock. This rock forms as the last remaining liquid, rich in rare metals and volatile substance, crystallizes after the cooling of great bodies of igneous rock. Presence of these volatiles and slow loss of heat to the surroundings allow giant crystals to form. Rarely, the process results in magnificent gem crystals projecting into clay-filled openings at the core of the pegmatite.
Reconstructed here is a tourmaline gem pocket discovered in the mountains of San Diego County, in southern California. Most of the clay filing the pocket when it was opened has been removed, so that the crystals appear in all their glory.