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.

Fabric is one way geologists describe how the small-scale components of a rock are arranged with respect to each other. Scientists look at grain size, grain shape, and the distribution of grain sizes within rocks to determine the fabric of those rocks. Sometimes the grains may be oriented randomly with respect to each other. The randomness may indicate that the rock may have formed at high temperatures (a rock called hornfels, for example) or from simple materials (marble, which results from the metamorphism of limestone, a sedimentary rock). Other times, the grains may be aligned in one direction at right angles to the direction of the pressure that helped to change (or metamorphose) the rock. Fabric is one of the many clues that help geologists identify rocks and learn about the history of those rocks.