Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None (conchoidal fracture); 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. Basalts can range from recent volcanic eruptions back to billions of years old.

Uses & Applications

Extensively used in construction for road base, concrete aggregate, railroad ballast, and floor tiles. High-quality samples are used as 'basalt fiber' for reinforcement.

Geological Facts

Basalt is the most common rock in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon, Mars, and Venus.

Field Identification & Locations

Identify by its dark color, heavy weight (density), and fine-grained texture with no visible crystals to the naked eye. Found worldwide in volcanic regions like Hawaii, Iceland, and the Deccan Traps.

Identified on: 4/15/2026

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