Basalt

Physical Properties

Hardness: 6 on Mohs scale; Color: dark grey to black; Luster: dull to earthy; Crystal structure: fine-grained (aphanitic) or porphyritic; Specific gravity: approx. 2.9-3.1

Formation & Geological History

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. Found in ocean basins, mid-ocean ridges, and volcanic arcs.

Uses & Applications

Used primarily in construction for crushed stone, road base, concrete aggregate, and railroad ballast. Also used as dimension stone and for cast basalt products.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up the majority of the ocean floor. It is also found extensively on the Moon and Mars.

Field Identification & Locations

Identify in the field by its very dark color, heavy weight (relative to its size), and lack of visible crystals without a hand lens. Commonly found near ancient volcanic sites or active ocean basins.

Identified on: 5/14/2026

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