Diorite

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Salt and pepper appearance (mottled black and white). Luster: Phaneritic (crystalline), dull to sub-vitreous. Structure: Medium to coarse-grained granular texture. Cleavage: Sub-conchoidal fracture. 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 between granite and gabbro, often found as large plutons or dikes.

Uses & Applications

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

Geological Facts

Ancient Egyptians often used diorite for statues and stelas because of its extreme hardness; the Code of Hammurabi was famously inscribed on a pillar of black diorite.

Field Identification & Locations

Identify by its 'salt and pepper' look; it contains more dark minerals than granite (like hornblende or biotite) and lacks the pink/red potassium feldspar and clear quartz associated with granite. Commonly found in the Andes Mountains and localized areas of the Appalachian Range.

Identified on: 4/17/2026

Mode: Standard