igneous
Diorite (Salt and Pepper Rock)
Phaneritic Plutonic Diorite (Plagioclase Feldspar + Amphibole/Biotite)
Hardness: 6-7 (Mohs scale); Color: Mottled black and white (salt and pepper); Luster: Sub-vitreous to dull; Crystal Structure: Phaneritic (coarse-grained coarse crystals visible to the eye); Cleavage: Good in two directions (feldspar component); Specific Gravity: 2.8 - 3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- Mottled black and white (salt and pepper)
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Mottled black and white (salt and pepper); Luster: Sub-vitreous to dull; Crystal Structure: Phaneritic (coarse-grained coarse crystals visible to the eye); Cleavage: Good in two directions (feldspar component); Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed from the slow cooling of intermediate magma beneath the Earth's surface in volcanic arcs and subduction zones. Most specimens range from the Proterozoic to Cenozoic eras.
Uses & applications
Used primarily in construction as crushed stone, 'black granite' for countertops, base material for roads, and occasionally in sculpture or as ornamental masonry.
Geological facts
Diorite was used by the Ancient Egyptians for statues because of its extreme hardness; the famous Code of Hammurabi was inscribed on a large black diorite stele.
Field identification & locations
Identify by its distinctive black-and-white speckled appearance and hard, interlocking crystals. Common in mountain ranges like the Andes and the Cascades; collectors should look for smooth, water-worn cobbles in riverbeds near volcanic regions.
