Basalt

Physical Properties

Hardness: 5-6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Microcrystalline (aphanitic); Cleavage: None/Irregular; 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 surface. This specimen shows water-worn rounding, suggesting finding in a river or coastal environment.

Uses & Applications

Commonly used as crushed stone for road construction, concrete aggregate, and railroad ballast. Fine-grained basalt is also used for decorative landscaping stones.

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 and Mars in large quantities.

Field Identification & Locations

Identify in the field by its dark color, fine-grained texture (crystals too small to see with the naked eye), and heavy feel for its size. Thin white lines may be quartz or calcite veins filling narrow fractures.

Identified on: 5/14/2026

Mode: Standard