Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Black or dark grey; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0

Formation & Geological History

Formed from the rapid cooling of low-viscosity 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 for aggregate, road base, railroad ballast, and dimension stone for buildings. It is also used as raw material for stone wool insulation.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars.

Field Identification & Locations

Identify in the field by its heavy weight, dark color, and fine-grained texture. It often weathers to a reddish-brown color due to iron oxidation. Look for it in volcanic plateaus and modern beach cobbles.

Identified on: 4/19/2026

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