Diorite (River Rock/Cobble)

Physical Properties

Hardness (Mohs): 6-7; Color: Black and white (salt and pepper appearance); Luster: Phaneritic (crystalline) and dull; Structure: Coarse-grained phaneritic; Cleavage: Good in two directions via feldspars; Specific Gravity: 2.7-3.3

Formation & Geological History

Formed from the slow cooling of magma beneath the Earth's surface in volcanic arches or subduction zones. This specific specimen has been smoothed by water action in a river or glacial environment.

Uses & Applications

Used primarily in construction as crushed stone for roads or as ornamental dimension stone for countertops and building facades. In smaller formats, used as landscaping stones or gravel.

Geological Facts

Diorite was used in ancient civilizations like Mesopotamia for high-end sculpture and tablets because of its extreme hardness; the famous Code of Hammurabi was inscribed on a diorite stele.

Field Identification & Locations

Identify in the field by the distinct 'salt and pepper' look where dark minerals (biotite/amphibole) and light minerals (plagioclase feldspar) are evenly mixed. Found in mountain ranges and glacial deposits across North America and Europe.

Identified on: 4/14/2026

Mode: Standard