Diorite (Salt and Pepper Rock)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Black and white (salt and pepper appearance); Luster: Subvitreous to dull; Crystal structure: Phaneritic (coarse-grained); Specific gravity: 2.8 - 3.0. Composed primarily of plagioclase feldspar and hornblende.

Formation & Geological History

Formed through the slow cooling of magma beneath the Earth's surface (intrusive). It typically occurs in volcanic arcs and mountain building areas (orogeny) where tectonic plates collide.

Uses & Applications

Used primarily in the construction industry as crushed stone for roads and drainage. In polished form, it is used for countertops, floor tiles, and as a decorative building stone known as 'black granite'.

Geological Facts

Diorite was used by ancient civilizations for sculpture because of its extreme hardness; 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, absence of visible quartz (unlike granite), and coarse grain. Commonly found in roots of mountain ranges like the Andes or the Alps.

Identified on: 4/24/2026

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