Basalt

Physical Properties

Hardness: 5-6 (Mohs); Color: Dark grey, black, or dark greenish; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0; often vesicular (pitted with holes).

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. Basaltic volcanism can occur in any geological age, commonly at mid-ocean ridges and hotspots.

Uses & Applications

Extensively used in construction as aggregate (crushed stone), road base, railroad ballast, and high-quality concrete. It is also used for floor tiles, monuments, and as 'lava stones' for massage or landscaping.

Geological Facts

Basalt is the most common rock type in the Earth's crust and covers most of the ocean floor. It is also extremely common on other planetary bodies like the Moon, Mars, and Venus.

Field Identification & Locations

Identify by its dark color, heavy weight (density), and fine-grained texture. It is commonly found in volcanic regions like Hawaii, the Pacific Northwest (Columbia River Basalt Group), and Iceland.

Identified on: 4/20/2026

Mode: Standard