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Medium- to coarse-grained igneous rock that commonly is composed of about two-thirds plagioclase feldspar and one-third dark-coloured minerals, such as hornblende or biotite. Diorite has about the same structural properties as granite but, perhaps because of its darker colour and more limited supply, is rarely used as an ornamental building material. It is one of the dark gray stones that is sold commercially as black granite.
This entry comes from Encyclopædia Britannica Concise. For the full entry on diorite, visit Britannica.com.