Diorite

Diorite is a coarse-grained plutonic rock of intermediate composition, made of roughly equal parts light plagioclase and dark hornblende or biotite, with little or no quartz. Its black-and-white "salt-and-pepper" look and its setting in volcanic arcs mark it as the intrusive equivalent of andesite.

🪨 Geology Guide Specimen: Open Diorite in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 7 subgroups of Igneous, and test your eye in LithoClade.

Diorite has far less quartz than granite, and its plagioclase is sodium-rich (andesine) rather than the calcium-rich labradorite of gabbro. The dark minerals are mostly hornblende and biotite, with pyroxene in smaller amounts, and they make up about 10 to 40 percent of the rock.

Diorite crystallizes in the roots of volcanic arcs, often alongside granodiorite and tonalite in the same batholiths. It represents magma that cooled underground rather than erupting, the plutonic counterpart of the andesite found in the volcanoes above.

Diorite is dense, tough and takes a high polish, so it was carved into statues and monuments in ancient times. Some famous "diorites," including the Egyptian stone of the statue of Khafre, have been reidentified as other rocks such as anorthositic gneiss, a reminder that archaeologists' stone names were often looser than modern ones.