A phaneritic (visibly crystallized) plutonic rock having intermediate SiO₂ content (53–66%), composed mainly of plagioclase (oligoclase or andesine) and one or more ferromagnesian minerals (hornblende, biotite, or pyroxene), and having a granular texture. Diorite is the plutonic equivalent of andesite (a volcanic rock). This dark gray rock is used occasionally as a building stone and is known commercially as black granite. For a general discussion of textural, structural, and compositional differences