Basalt (River Pebble)

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Dark grey to black. Luster: Dull or earthy when unpolished. Crystal structure: Aphanitic (fine-grained), often composed of pyroxene, plagioclase feldspar, and olivine.

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 coastal erosion during the Holocene era.

Uses & Applications

Used extensively in construction as aggregate, in road base, and concrete. In polished form, it is used for massage stones, landscaping, and decorative aquariums.

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 in abundance on the Moon and Mars.

Field Identification & Locations

Identify by its heavy weight (high density), dark color, and fine-grained texture. To find these, look in dried riverbeds or rocky beaches. Watch for a lack of visible large crystals compared to granite.

Identified on: 5/1/2026

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