Basalt

Physical Properties

Hardness: 5-6 Mohs. Color: Dark gray to black. Luster: Dull. Crystal structure: Aphanitic (fine-grained, crystals too small to see with the naked eye). Specific gravity: 2.8–3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near a planetary surface. Commonly found in oceanic crust and shield volcanoes.

Uses & Applications

Used primarily in construction for road base, concrete aggregate, railroad ballast, and dimension stone for pavers or countertops.

Geological Facts

Basalt is the most common rock on Earth's surface and also makes up a large portion of the surfaces of the Moon, Mars, and Venus.

Field Identification & Locations

Look for a heavy, dark, fine-grained rock that may have small holes (vesicles) or a smooth, worn surface if found in a river. It is common in volcanic regions and along coastlines.

Identified on: 4/15/2026

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