Basalt

Physical Properties

Hardness: 5-6 Mohs; Color: Dark grey to black, weathering to brown; Luster: Dull/Earthly to vitreous; Crystal structure: Aphanitic (fine-grained); Cleavage: None (conchoidal fracture); 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 the surface of a terrestrial planet or moon. Most basaltic rocks are less than 200 million years old.

Uses & Applications

Extensively used in construction as crushed stone for road bases, concrete aggregate, and railroad ballast. Also used as dimension stone for tiles and in the manufacture of rock wool insulation.

Geological Facts

Basalt is the most common rock type in Earth's crust and makes up the vast majority of the ocean floor. It is also common on the Moon, Mars, and Venus, forming the dark "seas" (maria) on the lunar surface.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (relative to size), and very fine-grained texture where individual crystals are not visible to the naked eye. Often occurs in columnar joints.

Identified on: 4/29/2026

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