Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to black; 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 at or very near the Earth's surface. Common in oceanic crust and volcanic regions.

Uses & Applications

Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality varieties are used for floor tiles 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.

Field Identification & Locations

Identify by its dark color, heavy weight (relative to size), and fine-grained texture that lacks visible crystals to the naked eye. Found worldwide in volcanic areas.

Identified on: 5/18/2026

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