Basalt (River Pebble)

Physical Properties

Hardness: 6; Color: Dark grey to dull greenish-grey; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); 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) erosion.

Uses & Applications

Used primarily in construction as aggregate, road base, and concrete; fine specimens are used in lapidary work or as decorative river stones.

Geological Facts

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

Field Identification & Locations

Identify by its dark color, fine-grained texture, and heavy weight for its size. High concentrations found in volcanic regions and riverbeds downstream from volcanic plateaus.

Identified on: 5/2/2026

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