Basalt

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Black, dark gray, or dark brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.9–3.1. It often exhibits conchoidal fractures.

Formation & Geological History

Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava (magma) at or very near the Earth's surface. This specimen appears water-worn, likely from a river or beach environment.

Uses & Applications

Extensively used in construction as aggregate for roads, concrete, and railroad ballast. Some dense varieties are used for floor tiles and monuments.

Geological Facts

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

Field Identification & Locations

Identify by its heavy weight (high density), dark color, and fine-grained texture. It usually lacks visible crystals unless it is porphyritic. This specific piece is rounded by erosion, showing typical pitting and a smooth, weathered surface.

Identified on: 4/19/2026

Mode: Standard