Diorite

Physical Properties

Hardness (Mohs): 6-7; Color: Mottled 'salt and pepper' (black and white/gray); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Specific gravity: 2.8 to 3.0. Prominently contains Plagioclase feldspar and Hornblende/Biotite.

Formation & Geological History

Formed from the slow cooling of magma beneath the Earth's surface in intrusive environments, typically above subduction zones at convergent plate boundaries. Most are associated with mountain building events across various geological eras.

Uses & Applications

Used primarily as crushed stone for road construction and aggregate. High-quality slabs are polished for use as countertops, floor tiles, and building veneers. Historically used in sculpture (e.g., the Code of Hammurabi stela).

Geological Facts

Ancient Egyptians used diorite to carve hard stones like granite because it is extremely tough. Its characteristic speckled 'salt and pepper' look is due to the roughly equal mix of light plagioclase and dark minerals.

Field Identification & Locations

Identify by looking for a coarse-grained texture where individual white/gray and black mineral crystals are visible to the naked eye. It lacks the pinkish potassium feldspar found in granite and contains more dark minerals than typical granite. Common in the Andes Mountains and parts of the UK and Italy.

Identified on: 4/23/2026

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