Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: dark gray to black; Luster: dull to sub-metallic; Crystal structure: aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8-3.0; Cleavage: none (conchoidal fracture).

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the surface of Earth. The rounded shape indicates extensive water erosion, likely in a river or coastal environment.

Uses & Applications

Crushed stone for road base, concrete aggregate, railroad ballast, and high-density industrial flooring. Polished specimens are used for massage stones or decorative landscaping.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up the majority of the ocean floor. It is also found on the Moon and Mars.

Field Identification & Locations

Identify in the field by its high density (feels heavy for its size), dark color, vesicles (if present), and fine-grained texture. It often shows white veins of secondary minerals like quartz or calcite filling cracks.

Identified on: 5/14/2026

Mode: Standard