Diorite

Physical Properties

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

Formation & Geological History

Formed by the slow cooling of magma beneath the Earth's surface in volcanic arches or mountain-building zones (subduction zones). It is intermediate in composition between granite and gabbro.

Uses & Applications

Commonly used as a base material in construction (crushed stone), road building, and as a decorative stone in architecture. Occasionally used for sculptures or kitchen countertops due to its durability.

Geological Facts

Black diorite was used by ancient civilizations for durable monumental carvings; the famous Code of Hammurabi was inscribed on a diorite stele over 7 feet tall. It is often confused with granite but lacks significant amounts of quartz.

Field Identification & Locations

Identify in the field by its distinctive 'speckled' black and white texture and coarse crystal size. Look for it in areas of ancient volcanic activity or uplifted continental crust. It will not scratch easily with a steel knife.

Identified on: 4/26/2026

Mode: Standard