Applied to any igneous rock, which has a high proportion of pyroxene and olivine, such that it's colour index is between 50 and 90 (i.e. dark).
Subject: The Earth An igneous rock composed chiefly (50%) of one or more ferromagnesian, dark-colored minerals (derived from magnesium and ferric).[ Pics List
used to describe rocks rich in mafic minerals which are dark colored minerals rich in iron and magnesium