Gabbro is an igneous rock made mostly of feldspar and pyroxene, plus a little bit of quartz. Feldspar is light in color, and pyroxene is almost black. Together, these minerals give gabbro its speckled look. Gabbro forms in thick sheets when large masses of magma slowly cool. Because it cools slowly, it has large crystals that are easy to see.
Compared to granite, gabbro is darker in color and heavier. Both its darkness and its heaviness are results of the pyroxene in gabbro. Most gabbro forms in Earth’s crust under the basalt of the ocean floors.
Some gabbro, however, forms on Earth’s continents. Some famous masses of gabbro are in Stillwater, Montana; in Bushveldt, in South Africa; and on the Isle of Skye, in western Scotland.