A rock is made of minerals that are stable in a given environment. If the physical conditions of a rock change, especially temperature and pressure, the minerals will not be stable. A new group of minerals will begin forming under metamorphism until the environment becomes stable again. These mineral assemblages will appear depending on conditions.

The initial composition of a rock is very important. This will decide what it will become with changing metamorphic conditions. A rock with the same composition can turn into a different metamorphic rock, depending on the pressure and temperature. If the pressure and temperature are the same, the initial rock control the path metamorphism takes.