Basalt

Physical Properties

Hardness: 6 (Mohs scale), Color: Dark grey to black, Luster: Dull/Sub-metallic, Crystal structure: Aphanitic (fine-grained/microscopic), Specific Gravity: 2.8 - 3.0

Formation & Geological History

Basalt is an extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. It can form in various geological ages and makes up most of the ocean floor.

Uses & Applications

Used widely in construction as an aggregate (crushed stone) for roads, railroad ballast, and concrete. It is also used as a dimension stone for floor tiles and in the production of stone wool insulation.

Geological Facts

Basalt is the most common rock type in the Earth's crust and also occurs extensively on the Moon (the dark lunar maria) and Mars. Some of the most famous formations are the Giant's Causeway in Northern Ireland and the Devils Postpile in California.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (relative to size), and very fine-grained texture where individual crystals are not visible to the naked eye. It commonly exhibits columnar jointing in large flows.

Identified on: 4/26/2026

Mode: Standard