Gabbro is a coarse grained, equigranular igneous rock composed mainly of plagioclase feldspar (labradorite or bytownite) and orthoclase, a pyroxene such as augite, sometimes also nepheline and olivine, and a large percentage of hornblende (in hornblende gabbro). Rocks of this type are sometimes known as essexite. Gabbro forms as a result of the slow and undisturbed cooling of magma (molten rock) deep underground.