igneous
Diorite
Phaneritic intermediate plutonic rock
Hardness: 6-7 on Mohs scale; Color: 'Salt and pepper' (black and white); Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (coarse-grained); Cleavage: Dependent on mineral (2 planes for feldspar); Specific Gravity: 2.8-3.0
- Hardness
- 6-7 on Mohs scale
- Color
- 'Salt and pepper' (black and white)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: 'Salt and pepper' (black and white); Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (coarse-grained); Cleavage: Dependent on mineral (2 planes for feldspar); Specific Gravity: 2.8-3.0
Formation & geological history
Formed by the slow cooling of intermediate magma underground, typically in volcanic arcs or subduction zones (e.g., above oceanic-continental boundaries).
Uses & applications
Extensively used as crushed stone for road construction and as dimension stone for building facades, floor tiles, and grave markers. Often sold as 'black granite' in the countertop industry.
Geological facts
Ancient Egyptians used diorite to carve statues and vessels because of its extreme toughness. The Code of Hammurabi was famously inscribed on a 7-foot tall diorite stele.
Field identification & locations
Identify by its phaneritic (visible crystals) coarse-grained texture and balanced mix of light plagioclase feldspar and dark minerals like hornblende or biotite. It lacks the significant quartz found in granite.
