Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. Most basalt flows on Earth have formed during the Cenozoic era, but can date back billions of years.

Uses & Applications

Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. It is also used as dimension stone and for industrial insulators.

Geological Facts

Basalt is the most common rock type in Earth's crust and makes up most of the ocean floor. It is also widely present on the Moon, Mars, and Venus, forming the lunar maria.

Field Identification & Locations

Identify by its dark color, heavy weight (density), and fine-grained texture lacking visible crystals. Commonly found in volcanic regions, oceanic island chains (like Hawaii), and mid-ocean ridges.

Identified on: 5/1/2026

Mode: Standard