Diorite (Salt and Pepper Rock)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Mottled black and white (salt-and-pepper appearance); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Good (due to feldspar and hornblende content); Specific Gravity: 2.8-3.0.

Formation & Geological History

Formed from the slow cooling of magma beneath the Earth's surface in intrusive environments, often associated with volcanic arcs and subduction zones. Typically formed during orogenic (mountain-building) events.

Uses & Applications

Used primarily as crushed stone for road construction and drainage. High-quality specimens are polished and used as 'black granite' for countertops, floor tiles, and architectural veneers.

Geological Facts

Diorite was used by ancient civilizations for sculpture and masonry because of its extreme hardness; the famous Code of Hammurabi was inscribed on a massive diorite stele.

Field Identification & Locations

Identify in the field by its distinctive coarse-grained, speckled appearance containing roughly equal amounts of light (plagioclase feldspar) and dark (hornblende/biotite) minerals. It is commonly found in the roots of mountain ranges.

Identified on: 4/25/2026

Mode: Standard