Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Fine-grained (Aphanitic); 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 at or very near the Earth's surface. Common in oceanic crust and volcanic regions; geological age varies from millions of years to recent volcanic activity.

Uses & Applications

Extensively used in construction for road base, concrete aggregate, railroad ballast, and decorative garden stone. Due to its durability, it is also used in high-strength floor tiles and stone carvings.

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 in the form of 'maria' or vast plains.

Field Identification & Locations

Identify by its dark color, heavy weight (density), and fine-grained texture lacking visible crystals to the naked eye. Often found in volcanic fields or as crushed gravel in driveways and landscaping.

Identified on: 4/19/2026

Mode: Standard