Basalt (River Pebble)

Physical Properties

Hardness: 5-6 Mohs. Color: dark gray, black, or earthy brown (weathered). Luster: dull to sub-metallic. Crystal Structure: fine-grained (aphanitic). Cleavage: non-existent. 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 surface of a terrestrial planet or the moon. Most basalt seafloors are younger than 200 million years.

Uses & Applications

Commonly used as crushed stone for road base, concrete aggregate, railroad ballast, and high-quality asphalt. It is also used in 'hot stone' therapies and stone wool insulation.

Geological Facts

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

Field Identification & Locations

Identify by its dark color, heavy weight (relative to size), and fine-grained texture. To identify in the field, look for a lack of visible large crystals and a tendency to weather into rounded shapes in riverbeds.

Identified on: 4/29/2026

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