Many of the important gem minerals seen on today's market - aquamarine, tourmaline, and topaz, among others - come from an unusual type of rock known as a pegmatite. Gem-bearing pegmatites are crystalline igneous rocks that are distinguished by their large-size crystals, concentrations of certain chemical elements otherwise rare in the earth's crust, and various unusual minerals. Pegmatites are typically rather small bodies of rock that are found in particular geologic environments; the gem minerals occur in open cavities or "pockets" within the pegmatite. This article surveys our current understanding of pegmatites, beginning with a brief description of their characteristics and following with a discussion of the occurrence of gem minerals in them. The article concludes with a summary of the specific conditions necessary during pegmatite formation for the crystallization of abundant gem minerals.