igneous
Diorite (Salt and Pepper Rock)
Phaneritic Plutonic Diorite; chemical composition roughly 52-63% silica (SiO2)
Hardness: 6-7 (Mohs scale); Color: Mottled black and white (speckled); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (medium to coarse-grained), primarily composed of Plagioclase Feldspar and Hornblende/Biotite; Cleavage: Good in feldspar crystals; Specific gravity: 2.8 - 3.0.
- Hardness
- 6-7 (Mohs scale)
- Color
- Mottled black and white (speckled)
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Mottled black and white (speckled); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (medium to coarse-grained), primarily composed of Plagioclase Feldspar and Hornblende/Biotite; Cleavage: Good in feldspar crystals; 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 at subduction zones. It often occurs as large intrusions call batholiths or smaller dikes. It is typically associated with various geological ages from the Archean to the Cenozoic.
Uses & applications
Used primarily in construction as crushed stone or 'road base.' High-quality polished specimens are used as 'black granite' for countertops, floor tiles, and building veneers. Historically used in carving, such as the Code of Hammurabi stele.
Geological facts
One of the most famous archaeological artifacts in history, the Stele of the Law Code of Hammurabi, was carved into a huge specimen of diorite. Because diorite lacks quartz, it is tougher and more resistant to weathering than granite.
Field identification & locations
Identify in the field by its distinctive 'salt and pepper' appearance and visible distinct mineral crystals. It is darker than granite but lighter than gabbro. Found in mountain-building regions like the Andes, the Cascades, and the Scottish Highlands.
