igneous
Diorite (Salt and Pepper Rock)
Diorite
Hardness: 6-7 (Mohs scale); Color: Black and white (salt and pepper appearance); Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (due to plagioclase and amphibole content); Specific gravity: 2.8 - 3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- Black and white (salt and pepper appearance)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Black and white (salt and pepper appearance); Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (due to plagioclase and amphibole content); Specific gravity: 2.8 - 3.0
Formation & geological history
Formed through the slow cooling of silicate magma underground in the Earth's crust (intrusive). It is often associated with volcanic arcs and subduction zones. Common geological ages vary from Precambrian to Cenozoic.
Uses & applications
Used primarily as a crushed stone for construction and road base. Due to its durability and ability to take a high polish, it is also used for countertops, floor tiles, and as a decorative stone in architecture and sculpture.
Geological facts
Diorite is often called 'Black Granite' in the commercial stone trade, though it lacks the quartz content of true granite. The Code of Hammurabi was famously inscribed on a pillar of black diorite.
Field identification & locations
Identify in the field by its speckled 'salt and pepper' look where light plagioclase feldspar meets dark hornblende or biotite. It lacks the visible quartz typical of granite. Commonly found in large batholiths or mountainous roots.
