Serpentinite forms by metamorphism of olivine-rich peridotites (dunites - ultramafic, 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, 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 alignment of crystals. The appearance of "foliated" serpentinites is really the result of extensive development of slickenlined surfaces.
Many serpentinites also have a small component of magnetite, but it's usually significant enough to feel a slight tug when a magnet is placed next to the rock.
Many Precambrian greenstone belts have significant occurrences of serpentinites. Slices of dunitic mantle caught up in orogenic belts by obduction (ophiolites) are often serpentinized. Many mantle peridotite masses that have been caught up in rising magmas have been serpentinized (for example, in kimberlites & lamproites).
Serpentinite (10.0 cm across) ("foliated") from Quebec's Thetford Mines Ophiolite Complex, representing ancient oceanic lithospheric peridotite that was metamorphosed during the Taconic Orogeny (Ordovician).
Locality: Thetford Mines area, southeastern Mégantic County, southern Quebec, Chaudiere Appalachians, south of the St. Lawrence River, southeastern Canada.
Serpentinite (3.6 cm across) ("foliated"), composed of picrolite antigorite serpentine, from a picrolite vein in metamorphosed Deer Lake Peridotite (late Neoarchean) in northern Michigan, USA.
Locality: Ropes Gold Mine, Ishpeming Greenstone Belt, Marquette County, western Upper Peninsula of Michigan, USA.
Serpentinite (9.6 cm across) (crystalline) from a metamorphosed body of oceanic lithospheric dunite (mantle peridotite) hosted in schists amd quartzites of the Ottauquechee Formation (Lower Cambrian). Metamorphism occurred during the Taconic Orogeny and Acadian Orogeny (Early Paleozoic & Middle Paleozoic).
Locality: J.A. Vermont Verde Antique International Quarry, eastern side of Quarry Hill Road, northeast of Rochester, northwestern Windsor County, Green Mountains central Vermont, USA (43° 54' 55" N, 72° 48' 26" W).