igneous
Diorite
Diorite (Intermediate Plutonic Rock)
Hardness: 6-7 (Mohs scale); Color: Salt and pepper (mottled black and white/grey); Luster: Phaneritic (medium to coarse-grained crystals), dull when weathered; Structure: Granular/Massive; Cleavage: Good in feldspar and amphibole grains.
- Hardness
- 6-7 (Mohs scale)
- Color
- Salt and pepper (mottled black and white/grey)
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Salt and pepper (mottled black and white/grey); Luster: Phaneritic (medium to coarse-grained crystals), dull when weathered; Structure: Granular/Massive; Cleavage: Good in feldspar and amphibole grains.
Formation & geological history
Formed from the slow cooling of magma beneath the Earth's surface in volcanic arcs and mountain-building zones (subduction zones). It is typically of Phanerozoic age but can be found in older cratons.
Uses & applications
Used primarily as crushed stone for road construction and drainage. High-quality specimens are used in architectural surfacing (countertops, tiles) and as a building stone known commercially as 'black granite'.
Geological facts
Diorite was used in ancient Egypt for statues and sarcophagi because of its extreme toughness. The Code of Hammurabi was famously inscribed on a pillar of black diorite.
Field identification & locations
Identify by its 'salt and pepper' appearance with roughly equal amounts of light (plagioclase feldspar) and dark (hornblende/biotite) minerals. Commonly found in coastal cobbles and mountainous terrains like the Andes or the Cascades.
