Gabbro is a dark plutonic rock crystallized from iron-rich magma at considerable depth beneath the surface, containing coarse, iron-bearing grains of calcium-rich plagioclase feldspar and pyroxene. In addition, the rock may contain olivine, hornblende, biotite, garnet, rutile, apatite, zircon, magnetite, ilmenite, and chromite. Chemically and mineralogically, gabbro is equivalent to basalt.
Gabbros may occur as borders on granitic and other plutons, or as individual plutons or dikes. They commonly occur in the lower layers of large complexes. Most gabbros appear to intrude the rocks surrounding them and so are thought to be of igneous origin, although some may also be produced by metamorphic processes.
Gabbro is lower in silica and darker in color than diorite, and chemically the equivalent of diabase.
I know it says "first draft," but this looks remarkably like the text at "http://www.geology.wisc.edu/~jill/gabbro.html" (I was looking for the chemical formula for gabbro). Epopt, are you ~jill? --Vicki Rosenzweig