Earth's crust, or outermost rocky layer, sits on top of a deeper layer called the mantle, which stores heat from two sources: the formation of the Earth 4.65 billion years ago and the radioactive decay of uranium, thorium, and potassium. When cracks between huge crustal plates open up, the gap causes the underlying mantle to rise up. The upwelling partially melts that region of the mantle; scientists call that decompression melting. The molten rock, or magma, is less dense than solid rock, so it moves upward, the way a cork bobs to the surface of water. As the magma reaches the upper layers of the crust or even Earth's surface, it cools and hardens into a solid known as igneous rock. Scientists categorize igneous rocks according to their chemical composition, the method of their formation, and their degree of crystallization.

Molten rock, or magma, does not always reach Earth's surface. It may flow upward through cracks that end below the surface, where it gets trapped and cools slowly. Some intrusions, called plutons, are several kilometers or miles wide. During the slow cooling process, the magma freezes into crystals. Magma containing higher iron, magnesium, and calcium levels is the first to turn solid and forms dark, coarse-grained rocks such as gabbro. The lighter-colored granite comes from magma with high levels of silica (silicon dioxide) and relatively little iron and magnesium. Humans cannot witness the formation of intrusive igneous rock in the same way we see volcano eruptions. However, over millions of years, the crustal rocks above some intrusions wear away, leaving the solidified magma exposed to the environment in places such as the Sierra Nevada Mountains and Yosemite National Park in California.

Pegmatites are extremely coarse-grained intrusive igneous rocks, containing crystals that are both large (at least 5 cm or 2 inches across) and packed closely together. Pegmatites crystallize during the final stages of granite formation. The same silicate minerals that form granite - quartz, feldspar, and mica - generally make up the bulk of pegmatites, too, but the individual minerals in pegmatites can be many centimeters or even several meters in diameter. Some pegmatites also contain less common minerals, such as garnet, albite, lepidolite, beryl, and fluorite. Geologists and miners sometimes find beautiful, gemstone-quality crystals of topaz, beryl (aquamarine), rose quartz, smoky quartz, and other minerals within pegmatites.