Wiktionary
serpentinite
n. (context geology English) a metamorphic rock composed of serpentine minerals formed by the hydration of ultramafic rocks
Wikipedia
Serpentinite
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 mineral alteration is particularly important at the sea floor at tectonic plate boundaries.