What does gabbro mean?
Gabbro is a coarse-grained igneous rock formed when magma rich in iron and magnesium cools slowly deep within the Earth's crust, allowing large, visible crystals of plagioclase feldspar and pyroxene to grow together into a dense, dark mass. Chemically, it is identical to basalt, the fine-grained volcanic rock that pours out at the surface; the difference between them lies purely in cooling rate rather than composition. Gabbro makes up a substantial portion of the lower oceanic crust and occurs in large intrusions on continents, some layered in striking bands like sedimentary strata. Because it is durable and takes a high polish, dark gabbro is widely quarried for gravestones, tiles, and kitchen counters, often sold under the misleading name 'black granite.' The word remains chiefly a technical term of geology, valued for the precision it lends to describing one of Earth's most abundant hidden rocks.
nounA coarse-grained, dark-colored intrusive igneous rock composed mainly of plagioclase feldspar and pyroxene, chemically equivalent to basalt but formed deep underground where slow cooling allows large crystals to develop.
- A coarse-grained intrusive igneous rock consisting chiefly of plagioclase feldspar and pyroxene, forming much of the lower oceanic crust.
"The geologist identified the dark, speckled outcrop as gabbro, noting its coarse interlocking crystals of pyroxene and feldspar."
"Massive bodies of gabbro were exposed when the quarry cut deep into the hillside."
"Polished slabs of black gabbro are often marketed commercially as 'black granite' for countertops."
The plural refers to multiple occurrences or varieties of the rock, typically in technical geological writing.
"The region contains several distinct gabbros, each recording a different episode of magma emplacement."
The rock beneath nearly every ocean floor is gabbro — you're likely standing closer to it right now than to any other rock on Earth.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of gabbro
The name gabbro was borrowed from Italian in the early nineteenth century, taking its name from the village of Gabbro in Tuscany, near Livorno, where the rock was originally described. The term entered scientific use through German geological literature before becoming established internationally as petrology developed as a discipline. Because the name is a toponym rather than a classical root, gabbro has few true cognates in English.
How gabbro is actually used
Gabbro is primarily a technical term used in geology, petrology, and related sciences, though it appears in commercial contexts such as stone quarrying and countertop sales. It carries no figurative or idiomatic usage. In the building-stone trade it is sometimes loosely sold under the name 'black granite,' which geologists consider a misnomer.
Easily confused with gabbro
Basalt is the fine-grained extrusive equivalent of gabbro — same chemistry, but cooled quickly at the surface so its crystals are too small to see.
Diorite is a similar coarse-grained intrusive rock but contains little or no pyroxene and is typically lighter in color than gabbro.