igneous
Diorite
Diorite, a coarse-grained intrusive igneous rock composed chiefly of plagioclase feldspar and hornblende, often with biotite or pyroxene
AI-generated identification · may be incorrectVisible interlocking black-and-white crystals give it a speckled appearance. Hardness is typically about 6–7 Mohs; luster is mostly vitreous on crystal faces, and it has no consistent cleavage as a whole rock.
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Physical properties
Visible interlocking black-and-white crystals give it a speckled appearance. Hardness is typically about 6–7 Mohs; luster is mostly vitreous on crystal faces, and it has no consistent cleavage as a whole rock.
Formation & geological history
Forms when intermediate-composition magma cools slowly beneath Earth’s surface, allowing large crystals to grow. Diorite occurs in ancient and modern continental crust; the specimen’s age cannot be determined from the image.
Uses & applications
Used as dimension stone, paving, aggregate, and occasionally decorative or sculptural stone; its mottled appearance also makes it useful for teaching and collecting.
Geological facts
Diorite is often called ‘salt-and-pepper’ rock. It is distinguished from granite by its lower quartz content and commonly darker overall color.
Field identification & locations
Look for coarse, interlocking light plagioclase and dark hornblende or biotite, with little or no visible quartz. It occurs in continental mountain belts and plutons worldwide, including the Andes and North American Cordillera.
