Basalt

Physical Properties

Hardness: 5-6 (Mohs); Color: Dark gray to black; Luster: Dull to sub-metallic (contains fine-grained silicates); 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 or very near the Earth's surface. Common in oceanic crust and volcanic regions.

Uses & Applications

Used extensively in construction as road base, railroad ballast, and concrete aggregate. Sometimes used as dimensional stone for flooring and monuments.

Geological Facts

Basalt is the most common rock type in the Earth's crust and also makes up most of the surface of the Moon and Mars. On Earth, it forms spectacular geometric columns like the Giant's Causeway.

Field Identification & Locations

Identify by its dark color, heavy weight (density), and lack of visible crystals to the naked eye. Often found in volcanic areas or coastal regions where lava flows met the sea.

Identified on: 4/26/2026

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