Basalt

Physical Properties

Hardness: 6.0 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: approx. 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 a moon. Most common rock in the Earth's crust.

Uses & Applications

Extensively used in construction for aggregate, road base, railroad ballast, and high-quality paving stones. Also used in the manufacture of stone wool insulation.

Geological Facts

Basalt makes up most of the ocean floors on Earth. It is also found on the Moon, Mars, and Venus. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt.

Field Identification & Locations

Identify by its dark color, heavy weight (relative to size), and fine-grained texture where individual crystals are too small to see with the naked eye. Found globally in volcanic regions.

Identified on: 4/15/2026

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