igneous
Diorite
Intermediate Phaneritic Igneous Rock
Hardness: 6-7 (Mohs scale); Color: 'Salt and pepper' (mottled black and white/gray); Luster: Subvitreous to dull; Crystal structure: Phaneritic (visible crystals); Specific gravity: 2.8-3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- 'Salt and pepper' (mottled black and white/gray)
- Luster
- Subvitreous to dull
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs scale); Color: 'Salt and pepper' (mottled black and white/gray); Luster: Subvitreous to dull; Crystal structure: Phaneritic (visible crystals); Specific gravity: 2.8-3.0
Formation & geological history
Formed from the slow cooling of magma beneath the Earth's surface in volcanic arches and mountain-building zones (subduction zones). It is intermediate in composition between granite and gabbro.
Uses & applications
Used as a base material in construction, crushed stone for roads, and occasionally as a polished decorative stone (called 'black granite' in the trade). Historically used for sculpture and tools.
Geological facts
The Code of Hammurabi, one of the oldest deciphered writings of significant length in the world, was inscribed on a 7-foot tall diorite stele. Its extreme hardness makes it difficult to carve.
Field identification & locations
Identify by its coarse-grained texture and balanced mix of light plagioclase feldspar and dark minerals like hornblende or biotite. It lacks the visible quartz common in granite. Found in large mountain ranges like the Andes.
