Gabbro is a basic igneous rock: if erupted on the surface,
the magma would form a basalt. When the magma cools and
crystallizes at depth, it forms a coarse-grained rock made
of interlocking crystals in which you can distinguish dark
and light minerals. The dark mineral is mostly pyroxene, and
the lighter one is plagioclase (calcium-sodium feldspar).
Gabbro generally does not contain quartz.