Diorite (River Cobble)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Salt and pepper (mottled white/gray and black); Luster: Dull to phaneritic (visible crystals); Crystal Structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (feldspar) and splintery (amphibole); Specific Gravity: 2.8-3.0.

Formation & Geological History

Formed through the slow cooling of magma beneath the Earth's surface in volcanic arcs and mountain-building zones. This particular specimen is a water-worn cobble, indicating it was transported and rounded by river action over thousands of years.

Uses & Applications

Commonly used as a base material in construction, road building (crushed stone), and occasionally as a decorative stone for countertops or drainage landscaping.

Geological Facts

Diorite was used by Ancient Egyptians for statues and vases because of its extreme hardness. The Code of Hammurabi was famously inscribed on a black diorite stele over 7 feet tall.

Field Identification & Locations

Identify by its 'salt and pepper' appearance with roughly equal parts light (plagioclase feldspar) and dark (biotite/hornblende) minerals. Found in riverbeds or glacial deposits where granitic basement rocks are exposed in mountains.

Identified on: 4/22/2026

Mode: Standard