Diorite. Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microclne and olivine. This sample has a prophyritic texture, porphyry being a general term for indicating igneous rocks that consist of large grains or crystals in a fine grained matrix. In this sample there is a matrix (light grey) of feldspar (andesine), containing grains of pyroxene and hornblende (dark grey).