igneous
Diorite
Diorite (Intermediate intrusive igneous rock)
Hardness: 6-7 Mohs; Color: Salt and pepper (mottled black and white); Luster: Dull to sub-vitreous; Structure: Phaneritic (coarse-grained crystals); Composition: Plagioclase feldspar and hornblende/biotite; Specific Gravity: 2.8-3.0
- Hardness
- 6-7 Mohs
- Color
- Salt and pepper (mottled black and white)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Salt and pepper (mottled black and white); Luster: Dull to sub-vitreous; Structure: Phaneritic (coarse-grained crystals); Composition: Plagioclase feldspar and hornblende/biotite; Specific Gravity: 2.8-3.0
Formation & geological history
Formed through the slow cooling of intermediate-composition magma beneath the Earth's surface, typically in volcanic arcs and mountain building zones (subduction zones). These rocks can date from various geological eras depending on regional tectonic history.
Uses & applications
Used primarily as crushed stone for road construction and drainage, or as dimension stone for building facades, pavers, and grave markers. Often sold as black granite in the countertop industry.
Geological facts
Diorite was used by ancient civilizations for sculpture because of its extreme hardness; the famous Code of Hammurabi was inscribed on a polished diorite stele. It is the intrusive equivalent of the volcanic rock andesite.
Field identification & locations
Identify by its coarse 'salt and pepper' appearance without visible quartz (unlike granite). Commonly found in the roots of mountain ranges such as the Andes, Cascades, and Alps. Look for large, interlocking crystals of white feldspar and dark minerals.
