Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: dark gray, black, or dark green; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) or porphyritic; Specific gravity: 2.8–3.0.

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. Basalt is terrestrial but also makes up most of the ocean floor and lunar maria.

Uses & Applications

Extensively used in construction as aggregate (crushed stone), road base, railroad ballast, and for high-durability floor tiles and building facades.

Geological Facts

Basalt is the most common rock type in the Earth's crust. Most of the ocean floor is made of basalt. Large basaltic formations like the Giant's Causeway in Ireland show unique hexagonal columnar jointing.

Field Identification & Locations

Identify by its dark color, heavy weight (relative to size), and very fine crystalline structure. Found globally in volcanic regions, oceanic islands, and mid-ocean ridges.

Identified on: 4/20/2026

Mode: Standard