Basalt

Physical Properties

Hardness: 6.0; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0; Cleavage: None.

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 in construction as road base, railroad ballast, concrete aggregate, and high-quality floor tiles. Often used in stonewashed denim production (pumice/basalt).

Geological Facts

Basalt makes up most of the Earth's oceanic crust and is also found on the Moon, Mars, and Venus. It is the most common volcanic rock on Earth.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (relative to size), and very fine-grained texture. Often found as rounded water-worn pebbles in rivers or coastal areas.

Identified on: 4/16/2026

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