Gabbro is a dark igneous rock. It is made of quite large crystals and has a speckled appearance.
Gabbro forms as large masses of magma cool, as granite does, but it is made of different minerals. Because it cools slowly, it has large, easily visible crystals.
Rather than occurring in batholiths, gabbro is commonly found in thick sheets of igneous rock.
Feldspar and pyroxene are the two main minerals in gabbro, but it also has a small amount of quartz-less than 10 percent.
Feldspar is a pale mineral, often occurring in thin crystals in gabbro, while pyroxene is almost black, giving the rock its speckled appearance. Compared with granite, gabbro is a very dark-colored rock, and is also noticeably heavier. This is because it contains a large amount of pyroxene, which is a dense mineral.
A mineral called olivine is sometimes found in gabbro. This is a green or brownish mineral rich in iron and magnesium, which crystallizes at very high temperatures.
Gabbro mainly forms in the Earth's crust beneath the basalts of the ocean floors but can also occur on continents. Granite typically occurs in a continental setting.
Some famous masses of gabbro are at Bushveldt in South Africa and Stillwater, Montana.
In Britain, the Cuillin Hills on the Isle of Skye, in western Scotland are largely made of gabbro, which weathers into jagged peaks.