Like picrite and peridorite, pyroxenite is an ultramafic rock formed from magmas that develop deep in the Earth's mantle. What makes pyroxenite different from these rocks - though similar to wehrlite and clinopyroxenite - is that it contains a high proportion of clinopyroxene (usually the mineral augite, clinopyroxenite) or orthopyroxene (enstatite or hypersthene, orthopyroxenite), at the expense of olivine. Sometimes, pyroxenites are inclusions in other magmas, and look rather like obsidian: shiny black and fracturing into the same sharp, conchoidal (curved) fragments. Pyroxenites rarely occur alone. Very often, they form layered complexes with other plutonic (deep-forming) igneous rocks such as gabbro and norite. In the Bushveld complex of South Africa, for instance, gabbro, norite and pyroxenite layers are interwoven, with more pyroxenite layers at the base and more gabbro layers at the top. Occasionally pyroxenite-like rocks may form when certain limestones are altered by contact with hot magma, but these are more properly called pyroxene hornfels.