Basalt

Physical Properties

Hardness: 6-7 (Mohs); Color: dark grey to black (weathering to brown/tan); Luster: dull; Crystal Structure: aphanitic (fine-grained); Specific Gravity: 2.8–3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. It is the most common rock in the Earth's crust, primarily forming oceanic plates and large volcanic flows through various geological eras.

Uses & Applications

Extensively used in construction as aggregate, road base, and concrete ballast. Sometimes used for floor tiles, monuments, and as 'hot stones' for massage therapies.

Geological Facts

Basalt is also found on the Moon and Mars. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt jointing.

Field Identification & Locations

Identify by its dark color, heavy weight, and fine-grained texture. It often shows tan or light brown weathering rinds (as seen in the image). Found in volcanic regions and coastal areas.

Identified on: 4/14/2026

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