igneous
Diorite
Intermediate plutonic igneous rock
Hardness: 6-7 Mohs; Color: Salt and pepper (black and white); Luster: Dull to phaneritic; Crystal Structure: Phaneritic (coarse-grained); Cleavage: Distinct in plagioclase grains; Specific Gravity: 2.8-3.0
- Hardness
- 6-7 Mohs
- Color
- Salt and pepper (black and white)
- Luster
- Dull to phaneritic
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Physical properties
Hardness: 6-7 Mohs; Color: Salt and pepper (black and white); Luster: Dull to phaneritic; Crystal Structure: Phaneritic (coarse-grained); Cleavage: Distinct in plagioclase grains; Specific Gravity: 2.8-3.0
Formation & geological history
Formed through the slow cooling of intermediate magma (between felsic and mafic) deep within the Earth's crust, often in volcanic arcs or mountain-building zones (subduction zones).
Uses & applications
Used primarily as crushed stone for road construction and drainage; occasionally used as ornamental stone, building cladding, or as 'black granite' in cemetery monuments and countertops.
Geological facts
Diorite was used by ancient civilizations for sculpture because of its extreme toughness; the famous Code of Hammurabi is inscribed on a black diorite stele.
Field identification & locations
Identify by its 'salt and pepper' appearance with roughly equal parts dark (hornblende/biotite) and light (plagioclase) minerals. Found in continental crust near plate boundaries like the Andes or Cascades.
