Diorite (Salt and Pepper Rock)

Physical Properties

Hardness (Mohs): 6-7; Color: Mottled black and white ('salt and pepper'); Luster: Dull to sub-vitreous; Structure: Phaneritic (coarse-grained) crystalline; Cleavage: Good (in feldspar/amphibole grains); 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 mountain-building zones. It is an intrusive rock, often associated with subduction zones. Can range from Proterozoic to Cenozoic in age.

Uses & Applications

Used primarily as a crushed stone for road construction and as a dimension stone for architecture, floor tiles, and grave markers. Occasionally used in jewelry as polished cabochons.

Geological Facts

Ancient Egyptians used diorite to carve statues because of its extreme hardness; the Code of Hammurabi was inscribed on a diorite stele. It is intermediate in composition between granite and gabbro.

Field Identification & Locations

Identify in the field by its distinctive 'salt and pepper' look where white plagioclase feldspar and dark minerals (hornblende/biotite) are roughly equal. Found in continental crust near plate boundaries.

Identified on: 4/24/2026

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