Diorite

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Salt and pepper (mottled grey, white, black). Luster: Dull to sub-vitreous. Structure: Phaneritic (coarse-grained). Cleavage: Present in amphibole/feldspar components. Specific Gravity: 2.8-3.0.

Formation & Geological History

Formed by the slow cooling of intermediate magma beneath the Earth's surface in volcanic arcs and mountain building zones (subduction zones). Usually Mesozoic to Cenozoic in age depending on the specific mountain range.

Uses & Applications

Used in construction as crushed stone or road base, as dimension stone for building facades, tiling, and occasionally for statues or cemetery markers. In ancient times, it was used for durable inscriptions.

Geological Facts

The Code of Hammurabi was inscribed on a massive black diorite stele. It is often called 'Black Granite' in the commercial stone trade, but geologically it contains little to no quartz.

Field Identification & Locations

Identify in the field by its coarse-grained 'salt and pepper' appearance and roughly equal mix of light plagioclase and dark minerals (hornblende/biotite). Common in the Andes, the Alps, and western North America.

Identified on: 5/3/2026

Mode: Standard