Serpentinites are rocks consisting mostly of the serpentine-group minerals chrysotile, lizardite and antigorite. They are formed by the hydration of olivine-rich ultramafic rocks and they contain up to ~13 wt% H2O. They have long been used by many cultures as building and carving stones. Serp entinites play essential roles in numerous geological settings. They act as a lubricant along plate boundaries during aseismic creep and contribute to the geochemical cycle of subduction zones. In the mantle, they are a reservoir of water and fluid-mobile elements. Serpentinites can produce nickel ore where weathered, and they can sequester CO2 where carbonated. They may have provided an environment for the abiotic generation of amino acids on the early Earth and other planets, potentially leading to the development of life.