Basalt

Physical Properties

Hardness (Mohs scale): 6; Color: dark gray to black; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) or porphyritic; Cleavage: none; Specific gravity: 2.8 - 3.0

Formation & Geological History

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic) at or very near the Earth's surface. Common in oceanic crust and large volcanic provinces.

Uses & Applications

Extensively used in construction, road base, concrete aggregate, railroad ballast, and sometimes as a dimension stone for floor tiles or monuments.

Geological Facts

Basalt is the most common rock in the Earth's crust and covers most of the ocean floor. It also makes up the lunar maria on the Moon.

Field Identification & Locations

Identify by its dark color, heavy weight (density), and fine-grained texture. It often shows vesiculation (small pits from gas bubbles). Commonly found in volcanic regions.

Identified on: 5/1/2026

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