Diorite

Physical Properties

Hardness: 6-7 (Mohs scale), Color: Mottled black and white ('salt and pepper'), Luster: Dull to sub-vitreous, Crystal structure: Phaneritic (coarse-grained), Specific gravity: 2.8-3.0.

Formation & Geological History

Formed by the slow cooling of magma deep underground, often in volcanic arcs or areas of mountain building (orogeny). It has an intermediate composition between granite and gabbro.

Uses & Applications

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

Geological Facts

A very famous specimen made of diorite is the Code of Hammurabi, a black diorite stele carved in ancient Mesopotamia. It is prized for its durability and ability to take a high polish.

Field Identification & Locations

Identify by its 'salt and pepper' appearance with roughly equal parts light (plagioclase feldspar) and dark (hornblende or biotite) minerals. Look for interlocking grains and a lack of quartz.

Identified on: 4/18/2026

Mode: Standard