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.

One clue to a mineral's identity is the way a sample breaks off a larger piece of mineral. If the mineral breaks to form fairly flat and smooth surfaces - planes of weakness in the crystal structure - geologists call it "cleavage." Mica and graphite have excellent cleavage because their atoms have strong bonds with each other within crystal planes, but only weak bonds between the planes. In other minerals, the atomic bonds have approximately the same strength in all directions, so they do not break into flat pieces when struck by a hammer. Scientists call this "fracture," and they use several adjectives to describe it. A "conchoidal" fracture has smooth, curved surfaces, while a "hackly" fracture is jagged, with sharp edges. Fracture may also be fibrous, splintery, or irregular.