Basalt River Cobble

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to charcoal/greenish-grey; Luster: Dull/Matte; Crystal structure: Fine-grained (aphanitic) or porphyritic; Specific Gravity: 2.8-3.0; Texture: Smooth, water-worn surface.

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 has been shaped and smoothed by fluvial or marine transport over hundreds to thousands of years.

Uses & Applications

Used primarily in construction as crushed stone, road base, or concrete aggregate. Smooth stones are also used in landscaping, river rock decor, and 'hot stone' massage therapy.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is the celestial equivalent of the 'maria' on the Moon.

Field Identification & Locations

Identify by its dark color, heavy weight (relative to size), and lack of visible large crystals. Common in volcanic regions and on beaches or riverbeds downstream from volcanic mountains.

Identified on: 5/6/2026

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