Gabbro refers to a large group of dark, coarse-grained, intrusive igneous rocks chemically equivalent to basalt. The vast majority of the Earth's surface is underlain by gabbro within oceanic crust, produced by basalt magmatism at mid-ocean ridges. Gabbro is dense, greenish or dark-colored and contains pyroxene, plagioclase, amphibole, and olivine (olivine gabbro when olivine is present in a large amount). The pyroxene is mostly clinopyroxene whereas small amounts of orthopyroxene may be present. If the amount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock is then a norite. Quartz gabbros are also known to occur and are probably derived from magma that was over-saturated with silica. Essexites represent gabbros whose parent magma was under-saturated with silica, resulting in the formation of the feldspathoid mineral nepheline. Gabbros contain minor amounts, typically a few percent, of iron-titanium oxides such as magnetite, ilmenite, and ulvospinel. Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finer grained equivalents of gabbro are called diabase, although the vernacular term microgabbro is often used when extra descriptiveness is desired. Gabbro may be extremely coarse grained to pegmatitic, and some pyroxene-plagioclase cumulates are essentially coarse grained gabbro, although these may exhibit acicular crystal habits. Gabbro is usually equigranular in texture, although it may be porphyritic at times, especially when plagioclase oikocrysts have grown earlier than the groundmass minerals.