Basalt

Physical Properties

Hardness: 6.0 on Mohs scale. Color: Black to dark gray. Luster: Dull to sub-vitreous. Crystal structure: Aphanitic (fine-grained), often containing plagioclase feldspar and pyroxene. 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 a moon. It is the most common volcanic rock on Earth, making up much of the ocean floor.

Uses & Applications

Used extensively in construction as an aggregate (crushed stone), for road base, railroad ballast, and in paving. Occasionally used as a polishing stone or for hot stone massage therapy.

Geological Facts

Basalt is found across the solar system, making up the lunar maria on the Moon and much of the surface of Mars and Venus. It is the primary component of the oceanic crust.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It often shows a smooth, rounded surface when eroded by water, as seen in this specimen. Found commonly in volcanic regions and ocean beaches.

Identified on: 4/19/2026

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