Diorite

Physical Properties

Hardness: 6-7 on Mohs scale; Color: 'Salt and pepper' (gray/white and black mix); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Specific gravity: 2.8-3.0.

Formation & Geological History

Formed by the slow cooling of magma beneath the Earth's surface in volcanic arches and mountain building zones. It is an intermediate intrusive rock, meaning it sits between granite and gabbro in composition.

Uses & Applications

Used primarily as crushed stone for road construction and drainage. It is also used as a decorative 'black granite' in counter tops and building facades when polished.

Geological Facts

Diorite is extremely tough; the Ancient Egyptians used it for sculpting because it is harder than granite. The Code of Hammurabi, one of the oldest law codes in history, was carved into a diorite stele.

Field Identification & Locations

Identify in the field by its distinctive black-and-white speckled appearance and hard, interlocking minerals (plagioclase and hornblende/biotite). Commonly found in the roots of mountain ranges.

Identified on: 4/23/2026

Mode: Standard