igneous
Diorite (River Rock)
Intermediate plutonic igneous rock
Hardness: 6-7 on Mohs scale. Color: 'Salt and pepper' (gray/white and black/dark green). Luster: Dull to sub-vitreous. Structure: Phaneritic (coarse-grained). Cleavage: Two directions (mostly from amphibole/feldspar). Specific gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- 'Salt and pepper' (gray/white and black/dark green)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: 'Salt and pepper' (gray/white and black/dark green). Luster: Dull to sub-vitreous. Structure: Phaneritic (coarse-grained). Cleavage: Two directions (mostly from 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 particular specimen is water-worn, smoothed into a cobble by river or glacial erosion over thousands of years.
Uses & applications
Used as crushed stone for road construction and drainage. High-quality diorite is sometimes polished as 'black granite' for countertops or cemetery monuments. Occasionally used for carvings.
Geological facts
Diorite was used by ancient civilizations for sculpture and tools due to its extreme hardness; the famous Code of Hammurabi was inscribed on a massive diorite stele.
Field identification & locations
Identify in the field by its speckled 'salt and pepper' appearance and large visible mineral crystals. Unlike granite, it lacks significant quartz. Commonly found in areas with tectonic or volcanic history.
