Diorite

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Spotted 'salt and pepper' appearance (black and white/gray). 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 beneath the Earth's surface in volcanic arcs and mountain-building zones (subduction zones). It is intermediate in composition between granite and gabbro.

Uses & Applications

Used primarily as crushed stone for road construction and drainage. High-quality specimens are polished for use as 'black granite' in countertops, floor tiles, and cemetery monuments.

Geological Facts

In antiquity, particularly in Ancient Egypt and Mesopotamia, diorite was highly prized for sculpture because of its extreme hardness, making it difficult to carve but very durable. The Code of Hammurabi was inscribed on a diorite stele.

Field Identification & Locations

Identify in the field by its distinctive black-and-white speckled appearance and medium-to-coarse mineral grains. It lacks the pink potassium feldspar found in granite and the higher quartz content of granodiorite.

Identified on: 4/24/2026

Mode: Standard