Basalt (River Cobble)

Physical Properties

Hardness: 5-6 (Mohs); Color: Dark grey to black with brownish oxidation spots; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None/Conchoidal fracture; Specific gravity: 2.8-3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specific specimen shows evidence of fluvial erosion (rounding by water transport) and chemical weathering.

Uses & Applications

Commonly used as crushed stone for road construction, concrete aggregate, and railroad ballast. Exceptionally smooth specimens are used in landscaping or massage therapy.

Geological Facts

Basalt is the most common rock in Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars.

Field Identification & Locations

Identify by its dark color, heavy weight (density), and fine-grained texture. Field test for magnetism as it often contains magnetite. Commonly found in volcanic regions or riverbeds downstream from volcanic sources.

Identified on: 4/30/2026

Mode: Standard