Diorite (River Pebble)

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (mottled grey, white, and black). Luster: Dull (weathered) to vitreous (on fresh surfaces). Structure: Phaneritic (coarse-grained). Cleavage: Imperfect. 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. This specific specimen has been rounded and smoothed by water erosion in a river or beach environment.

Uses & Applications

Used primarily in construction as crushed stone or 'road metal.' High-quality polished diorite is used for countertops, floor tiles, and as a dimension stone in architecture. Historically used for carving (e.g., the Code of Hammurabi).

Geological Facts

Diorite is often called 'Black Granite' in the commercial stone trade, though geologically it lacks enough quartz to be true granite. Its extreme hardness made it a favorite medium for ancient Egyptian and Mesopotamian sculptors.

Field Identification & Locations

Identify by its 'salt and pepper' appearance and lack of visible quartz compared to granite. It is commonly found in roots of mountain ranges (plutons) and frequently appears as smooth pebbles in riverbeds drainage from those mountains.

Identified on: 4/20/2026

Mode: Standard