igneous
Diorite
Intermediate plutonic igneous rock
Hardness 6-7 on the Mohs scale, phaneritic (coarse-grained) texture with a distinctive 'salt and pepper' appearance. Composed mainly of plagioclase feldspar, hornblende, biotite, and minor pyroxene or quartz. Dull to vitreous luster, specific gravity 2.8-3.0.
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Physical properties
Hardness 6-7 on the Mohs scale, phaneritic (coarse-grained) texture with a distinctive 'salt and pepper' appearance. Composed mainly of plagioclase feldspar, hornblende, biotite, and minor pyroxene or quartz. Dull to vitreous luster, specific gravity 2.8-3.0.
Formation & geological history
Forms by the slow crystallization of intermediate magma beneath the Earth's surface, often in subduction zones, volcanic arcs, and mountain-building regions (orogenic belts).
Uses & applications
Used widely as crushed stone for construction, road base, aggregate, and as a durable dimension stone for building facades, paving, and monuments (often sold commercially as 'black granite').
Geological facts
Ancient civilizations, including the Egyptians and Mesopotamians, used diorite for sculptures and stelae because of its extreme hardness and durability. The famous Code of Hammurabi is carved into a massive diorite stele.
Field identification & locations
Identified in the field by its characteristic coarse black-and-white speckled pattern, absence or near-absence of visible quartz, and high hardness. Commonly found in mountainous batholiths worldwide, such as the Andes, Cascades, and European Alps.
