Gabbro is a coarse-grained, intrusive igneous rock, the main minerals being plagioclase (i.e. a mixture of calcium and sodium feldspar), olivine, pyroxene. The essential minerals are dark apart from some of the feldspars and therefore the rock is usually dark or greenish.

Gabbro forms deeper in the earth's crust than basalt and occurs as batholiths and laccoliths. These are often found along mid-ocean ridges or in ancient mountains composed of compressed oceanic crust. In Iceland, there are a few small batholiths or laccoliths and then often with granophyre.

Information provided by Patrick M. Colgan, Northeastern University
http://www.casdn.neu.edu