Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark gray to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8–3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or moon. Most typically associated with oceanic ridges and hotspot volcanoes.

Uses & Applications

Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. It is also used for floor tiles and as a component in mineral wool insulation.

Geological Facts

Basalt is the most common rock type in the Earth's crust, covering most of the ocean floor. It is also extremely common on the Moon and Mars, where it forms the dark 'seas' known as lunar maria.

Field Identification & Locations

Identify by its dark color, heavy weight (relative to size), and fine-grained texture with no visible individual crystals unless they are phenocrysts. Found globally in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.

Identified on: 4/25/2026

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