Gabbro. Credit: Mineral Information Institute.
Gabbro is a dense, dark, course-grained igneous rock consisting largely of plagioclase feldspar, pyroxene, and olivine. It is the intrusive (i.e., cooled and solidified underground) equivalent of basalt. Accessory minerals in gabbros include apatite, hornblende, ilmenite, and magnetite.
A gabbro containing nickel at Sudbury in Canada is one of the richest sources known of that metal.