Gabbro "eyes-chain" from Germany.

Gabbro (pronunciation: /ˈɡæbroʊ/) refers to a large group of dark, often phaneritic (coarse-grained), mafic intrusive igneous rocks chemically equivalent to basalt. It forms when molten magma is trapped beneath the Earth's surface and slowly cools into a holocrystalline mass. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term "gabbro" may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". The term "gabbro" was used in the 1760s to name a set of rock types that were found in the ophiolites of the Apennine Mountains in Italy. Then, in 1809, the German geologist Christian Leopold von Buch used the term more restrictively in his description of these Italian ophiolitic rocks. He assigned the name "gabbro" to rocks that geologists nowadays would more strictly call "metagabbro" (metamorphosed gabbro). Von Buch named gabbro after Gabbro, a village in Rosignano Marittimo municipality of Tuscany. Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finer grained equivalents of gabbro are called diabase (also known as dolerite), although the 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, some may exhibit acicular crystal habits.