Serpentinite from the Precambrian of Wyoming, USA. (public display, Colorado School of Mines Geology Museum, Golden, Colorado, USA)

Serpentinite forms by metamorphism of olivine-rich peridotites (dunites - ultramafic, phaneritic, intrusive igneous rocks). Metamorphism of olivine in the presence of water results in the formation of the mineral serpentine (Mg3Si2O5(OH)4). A metamorphic rock composed principally of serpentine is thus a serpentinite.

Serpentinite has a mottled greenish color, often has the look & feel of hard candle wax, and ranges in texture from crystalline to “foliated”. Many serpentinites have a foliated look to them, but it’s really not due to an planar alignment of crystals. The appearance of “foliated” serpentinites is really the result of extensive development of slickenlined surfaces.

Serpentinite can have a small component of magnetite that is usually significant enough to feel a slight tug when a magnet is placed next to the rock. Some serpentinites (see above) have "veins" of white asbestos (= chrysotile serpentine).

Many Precambrian greenstone belts have significant occurrences of serpentinites. Slices of dunitic mantle caught up in orogenic belts by obduction (= ophiolites) are often serpentinized. Sometimes, mantle peridotite masses that were caught up in rising magmas have been serpentinized (for example, in kimberlites & lamproites).