Gabbro

Gabbro is an igneous rock that has crystallized deep in the Earth. Since the rock cooled and hardened (and crystallized) deep below the Earth’s surface, it will be coarse-grained. High pressure is usually found deep below the surface of the Earth. Here, molten material cools very slowly. The igneous rocks produced have large crystals.

Gabbro has the same mineral composition as basalt (olivine and pyroxene with smaller amounts of feldspar and mica), though basalt cools quickly above the Earth’s surface from lava. Gabbro is coarse-grained while basalt is fine-grained.

Gabbro is sometimes called “black granite” for it too is coarse-grained, but a large proportion of iron-bearing minerals make gabbro heavier and dark-colored. Gabbro can also be gray or light green.

Gabbro is widely used as crushed stone for concrete aggregate, road base material, and railroad ballast. Smaller quantities are cut and polished for dimension stone and called black granite.

See dimension stone, granite, crushed stone