Diorite

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (mottled black and white). Luster: Dull to sub-vitreous. Structure: Phaneritic (coarse-grained). Cleavage: Variable based on mineral content (amphiboles and plagioclase). Specific gravity: 2.8-3.0.

Formation & Geological History

Formed through the slow cooling of magma beneath the Earth's surface in volcanic arcs and at subduction zones. It represents an intermediate composition between granite and gabbro.

Uses & Applications

Used primarily as crushed stone for road construction and drainage. High-quality varieties are used as dimension stone for buildings, countertops, and floor tiles. Historically used for sculpture and inscriptions (e.g., the Code of Hammurabi).

Geological Facts

Diorite was one of the few stones chosen by ancient Egyptians and Mesopotamians for important statues due to its extreme hardness and durability. It is often referred to as 'black granite' in the commercial stone trade.

Field Identification & Locations

Identify in the field by its distinctive 'salt-and-pepper' look with roughly equal parts dark and light minerals and no visible quartz. Commonly found in large mountain ranges like the Andes or the Cascades.

Identified on: 4/14/2026

Mode: Standard