Serpentinite

Serpentinite is a rock composed of minerals of the serpentine group such as antigorite, chrysotile and lizardite, formed by serpentinization, a process of hydration and metamorphic transformation of rock consisting almost entirely of ferromagnetic silicate minerals, such as olivine and pyroxenes ( usually ultramafic igneous rocks in particular the peridotite family). Accessory minerals such as chlorite, talc and magnetite may be present. The name derives from the similiarity of the texture of the rock to that of the skin of snakes. The minerals of the serpentine group, rich in magnesium and water, give these rocks a color ranging from light green to intense dark green. There is never crystal in distinct forms; the structure of the aggregates is sometimes lamellar, fibrous; very fine aggregates with intertwined lamellas or fibrils are common, absolutely compact with the naked eye.
Serpentinites are widespread in Italy in the Alps and in the northern Apennines. Other deposits are located in California, Austria, Switzerland, Greece and Turkey.
Serpentinites with high concentrations of calcite have historically been used as ornamental stones for their marble-like qualities.