Basalt Pebble

Physical Properties

Hardness: 5-6 (Mohs). Color: Dark brown to black. Luster: Dull/Earthy. Structure: Fine-grained (aphanitic) with small vesicles (gas pits). 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 specimen has been rounded by fluvial (river) or coastal (wave) erosion.

Uses & Applications

Basalt is used in road base, concrete aggregate, railroad ballast, and for stone masonry. Smooth pebbles are often used in decorative landscaping and aquarium decor.

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 in the field by its dark color, heavy weight (density), and fine-grained texture. To find these, look near volcanic regions or in riverbeds downstream from volcanic mountains.

Identified on: 4/28/2026

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