Basalt

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Dark grey to black, often weathering to brown/green; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); 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.

Uses & Applications

Extensively used in construction for road base, concrete aggregate, railroad ballast, and as a raw material for stone wool insulation.

Geological Facts

Basalt underlies more of Earth's surface than any other rock type; most areas within Earth's ocean basins are underlain by basalt. It also makes up large portions of the Moon's surface.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (density), and lack of visible large crystal grains without a lens. Found globally in volcanic regions and ocean floor crust.

Identified on: 5/13/2026

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