High pressures and temperatures within the Earth's crust can change the textures and chemistry of minerals. In some cases, the high temperatures and pressures force traces of water or carbon dioxide out of a mineral, transforming it into a less hydrated species. Sometimes a mineral will not change its chemical composition during metamorphism, but its crystal structure will change into a more compact arrangement. For example, extreme heat and pressure can convert plain carbon from dull graphite, which has a flat, hexagonal crystal structure, to brilliant diamond, which has a closely packed, cubic crystal structure. Scientists say a metamorphic reaction is isochemical if minerals have the same amounts of chemical elements before and after the reaction (iso comes from the Greek word for equal).

Inside a mineral, atoms arrange themselves into a specific, repeating pattern called a crystal lattice or crystal structure. The smallest three-dimensional arrangement within the lattice is called a "unit cell," which is duplicated over and over again symmetrically. At the level of the everyday world, minerals that are growing without outside interference tend to form crystals that resemble their underlying crystal structures. Scientists call that kind of general, typical appearance a "crystal habit." Of course, conditions that existed during a mineral's formation or crystal growth may change its habit, but geologists still find this attribute to be a useful tool for identifying minerals. Scientists use more than three dozen adjectives to describe crystal habits. For example, natrolite and rutile can be acicular, or needlelike; quartz often forms hexagonal prisms; pyrite and halite typically crystallize as cubes; and mica is foliated or lamellar (layered).