Basalt (River Pebble)

Physical Properties

Hardness: 6.0 on Mohs scale; Color: Dark grey to black; Luster: Dull/Sub-metallic (when wet); 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 the earth's surface. This specimen has been rounded by fluvial (river) or coastal erosion processes.

Uses & Applications

Used in construction as aggregate, road base, and high-quality railroad ballast. Polished stones are used in decorative landscaping and 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 also found on the Moon and Mars.

Field Identification & Locations

Identify by its heavy weight (high density), dark matte color, and lack of visible crystals to the naked eye. Commonly found on volcanic beaches and in riverbeds flowing from volcanic highlands.

Identified on: 5/14/2026

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