Basalt (River Pebble)

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Dark grey to black, occasionally greenish; Luster: Dull/Matte; Structure: Fine-grained (aphanitic); Cleavage: None; 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 has been rounded by fluvial (river) or glacial erosion.

Uses & Applications

Used widely in construction as aggregate, for road base, railroad ballast, and in high-strength concrete. As a pebble, used for decorative landscaping.

Geological Facts

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

Field Identification & Locations

Identify in the field by its dark color, heavy weight for its size, and the absence of visible large crystals. Commonly found in volcanic regions or in riverbeds downstream from volcanic sources.

Identified on: 4/23/2026

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