Unlike granite, gabbro is low in silica and has no quartz; also gabbro has no alkali feldspar, only plagioclase that often is dark with a high calcium content. The other dark minerals may include amphibole, pyroxene and sometimes biotite, olivine, magnetite, ilmenite and apatite. This particular gabbro is mostly hornblende, magnetite and light-colored plagioclase.
Gabbro is named after a town in Tuscany, Italy. You can get away with calling almost any dark, coarse-grained igneous rock gabbro.
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