Gabbro is a dark, coarse-grained, intrusive igneous rock chemically equivalent to basalt. It is a plutonic rock, formed when molten magma is trapped beneath the Earth’s surface and cools slowly into a hard, coarsely crystalline mass. It is dense, greenish or dark-colored and contains varied percentages of plagioclase feldspar, pyroxene, amphibole, and olivine (called olivine gabbro when olivine is present in large quantities). Quartz gabbros are also known to occur and are probably derived from magma that was oversaturated with silica. On the other hand, essexites represent gabbros whose parent magma had an insufficiency of silica, resulting in the formation of nephelite.

Gabbro is too fragile to use in construction, but often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides.

Gabbro was named by the German geologist Christian Leopold von Buch after a town in the Italian Tuscany region.