Pegmatites are fascinating geologic features composed of gigantic mineral crystals. The conditions which favor their formation within magma are pretty specific: low nucleation rates (few sites for new crystals to grow), high diffusivity (which promotes rapid growth of crystals), high concentrations of water (which promotes high diffusivity) and a host of other features that contribute to the formation of few, large crystals. Pegatites are almost always concentrically zoned, with cores of quartz (such as above) and an alkali feldspar, either orthoclase or microcline. Surrounding the cores are zones composed of quartz, feldspar, and sometimes mica rounding out the major constituents. Many other minerals can be present, but are almost always in low quantities.
Pegmatites are economically important because they often contain small amounts of rare-earth elements and trace elements that form gemstones. The rare minerals that form in pegmatites are fascinating to both mineral collectors and mineralogists; so much that I have a couple of friends who may have co-discovered a cool new mineral in the location above. I expect a journal publication in the near future.
(Special thanks to Dr. Sarah Hanson, mineralogist extraordinaire, for offering input on this article.)