igneous
Diorite
Diorite, a coarse-grained intrusive igneous rock composed mainly of plagioclase feldspar and hornblende, often with pyroxene or biotite
AI-generated identification · may be incorrectTypically a salt-and-pepper mix of light plagioclase and dark amphibole or pyroxene; crystals are visible and interlocking, with a granular texture. Hardness is roughly 6–7 Mohs; luster is mostly dull to vitreous on fresh crystal faces.
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Physical properties
Typically a salt-and-pepper mix of light plagioclase and dark amphibole or pyroxene; crystals are visible and interlocking, with a granular texture. Hardness is roughly 6–7 Mohs; luster is mostly dull to vitreous on fresh crystal faces.
Formation & geological history
Forms when intermediate-composition magma cools slowly beneath Earth’s surface, allowing coarse crystals to develop. Diorite occurs in plutons and mountain-building belts of many geological ages.
Uses & applications
Used as crushed aggregate, road stone, and dimension stone; polished varieties have been used for monuments and decorative surfaces.
Geological facts
Diorite’s contrasting black-and-white appearance is often called “salt and pepper.” It can resemble granite, but usually contains little or no quartz and has darker minerals such as hornblende.
Field identification & locations
Look for coarse, interlocking crystals and abundant dark minerals alongside white feldspar; unlike granite, it is generally quartz-poor. Common in continental mountain belts and intrusive complexes; ordinary hand specimens typically sell for about $5–$30, with larger or polished pieces costing more.
