Basalt

Physical Properties

Hardness: 6 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained crystals generally not visible to the naked eye); 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 surface of a planet or moon. Basalt covers about 70% of Earth's surface, most of it on the ocean floor.

Uses & Applications

Major use in construction as aggregate (crushed stone), road base, railroad ballast, and in some architectural stone for flooring or monuments.

Geological Facts

Basalt is the most common rock type in the Earth's crust and also makes up the lunar mare (the dark plains on the Moon). It can sometimes form striking hexagonal columns, such as at the Giant's Causeway in Ireland.

Field Identification & Locations

Identify by its heavy weight (for its size), dark color, and very fine-grained texture. It often shows a weathered 'rind' that is brown or reddish due to iron oxidation. It is common in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.

Identified on: 4/29/2026

Mode: Standard