Gabbro is a dark colored plutonic rock composed mainly of coarse grains of calcium rich plagioclase feldspar and pyroxene. In addition, the rock may contain olivine, hornblende, biotite, garnet, rutile, apatite, zircon, magnetite, ilmenite, and chromite. Chemically and mineralogically, gabbro is equivalent to basalt. Gabbros may occur as border rocks around granitic and other plutons, or as small individual plutons or dikes. Their most common occurrence is in the lower parts of large, layered complexes. Most gabbros appear to intrude the rocks surrounding them and so are thought to be of igneous origin, although some may also be produced by metamorphic processes. It is lower in silica and darker in color than diorite, and chemically the equivalent of diabase.