gabbro

thumb|Gabbro specimen from Duluth, Minnesota thumb|Photomicrograph of a [[thin section of gabbro]]

Gabbro ( ) is a phaneritic (coarse-grained), mafic (magnesium- and iron-rich), intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro has the same chemical composition and mineralogy as rapid-cooling, fine-grained basalt. 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". It is often called "black granite" commercially, just as other intrusive rocks are called "granite" in the dimension stone industry. By rough analogy, gabbro is to basalt as granite is to rhyolite.