Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle. The alteration is particularly important at the sea floor at tectonic plate boundaries.

Serpentinization is a geological low-temperature metamorphic process involving heat and water in which low-silica mafic and ultramafic rocks are oxidized (anaerobic oxidation of Fe²⁺ by the protons of water leading to the formation of H₂) and hydrolyzed with water into serpentinite. Peridotite, including dunite, at and near the seafloor and in mountain belts is converted to serpentine, brucite, magnetite, and other minerals — some rare, such as awaruite (Ni₃Fe), and even native iron. In the process large amounts of water are absorbed into the rock increasing the volume and destroying the structure.

The density changes from 3.3 to 2.7 g/cm³ with a concurrent volume increase of about 40%. The reaction is exothermic and large amounts of heat energy are produced in the process.