High pressures and temperatures within the Earth's crust can change rocks from one type to another. Scientists call this process metamorphism, which comes from the Greek words for "after" and "form." At the junctions where the Earth's crustal plates collide, pushing up mountain ranges, or where one plate slides underneath another, the original rocks, or protoliths (from the Greek words meaning "first" and "rock") undergo reactions that change the chemical or crystal structure of the rocks with little or no actual melting. The grains of minerals within many metamorphic rocks are aligned in parallel due to the forces pushing on them. The pressure-temperature combination causes some sedimentary rocks to lose water molecules or become anhydrous.

The elements in metamorphic rocks do not change due to high temperatures and pressures. Iron will remain iron, silicon will still be silicon, and so forth. The basic chemical composition of the rocks remains the same before and after metamorphism. For example, limestone is a sedimentary rock made largely of calcium carbonate; the right conditions of high temperatures and pressures, usually found in mountain-building regions, can recrystallize the minerals in limestone to make marble, which also consists mostly of calcium carbonate. Likewise, sandstone, a sedimentary rock made from sand-sized grains of quartz, can metamorphose into quartzite, a hard rock consisting of interlocking crystals of quartz.

Many igneous and sedimentary protolith rocks that are the precursors to metamorphic rocks contain grains in sizes ranging from pebbles to nearly invisible silt particles. As underground pressures and temperatures increase, grains become compacted together, eliminating the pore spaces between the grains and sometimes making it hard for humans to see the grains at all without magnification. On the other hand, some metamorphic rocks still have grains that are visible to the human eye. Slate and phyllite are two types of fine-grained metamorphic rock. Gneiss (pronounced "nice") has medium to coarse grains and may resemble the igneous rock granite. Geologists use grain sizes and colors to help them identify different types of metamorphic rocks in the field.

Humans have long prized granite, an igneous rock rich in the minerals quartz and feldspar, as a building material because it does not crack easily and can be polished. The Smithsonian's National Museum of Natural History building was built out of four different types of granite from four different states on the East Coast of the United States. Other rocks that make up the building include roof tiles of slate, and museum floors made of terrazzo, which is crushed marble and granite with cement filler. Elsewhere in Washington, D.C., marble, granite, and bluestone gneiss went into the Washington Monument; marble was used inside and outside the Lincoln Memorial; and red sandstone makes up the Smithsonian Castle on the other side of the National Mall. The nearby National Archives, Department of Justice, and Internal Revenue Service buildings have limestone facades and granite foundations.