igneous
Diorite (River Cobble)
Phaneritic intermediate plutonic rock
Hardness: 6-7 Mohs; Color: Salt-and-pepper (mottled grey, white, and black); Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (medium-grained); Cleavage: Good in two directions (amphibole/feldspar); Specific gravity: 2.8-3.0
- Hardness
- 6-7 Mohs
- Color
- Salt-and-pepper (mottled grey, white, and black)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Salt-and-pepper (mottled grey, white, and black); Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (medium-grained); Cleavage: Good in two directions (amphibole/feldspar); Specific gravity: 2.8-3.0
Formation & geological history
Formed by the slow cooling of magma beneath the Earth's surface in volcanic arcs and mountain-building zones. This specific specimen has been rounded and smoothed by water transport in a river or coastal environment.
Uses & applications
Used as crushed stone for road construction, base material, and occasionally as 'black granite' for countertops or cemetery markers when polished.
Geological facts
Diorite was used in ancient Egypt and Mesopotamia for sculptures because of its extreme hardness; the Code of Hammurabi was inscribed on a diorite stele.
Field identification & locations
Identify by its 'salt and pepper' appearance with roughly equal parts dark and light minerals. It lacks the pink potassium feldspar found in granite and the higher quartz content. Found commonly in areas of continental crust subduction.
