Basalt

Physical Properties

Hardness: 6 Mohs; Color: Black to dark gray; Luster: Dull to sub-metallic (when polished); Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8 - 3.0

Formation & Geological History

Formed through the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of Earth. This specimen shows river-worn rounding from mechanical weathering over time.

Uses & Applications

Commonly used as aggregate in construction, road base, concrete, and decorative landscaping stones. Highly polished basalt is sometimes used in jewelry or as massage 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 extremely common on the Moon and Mars.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (density), and lack of visible crystals. This specific piece is a 'river rock,' identified by its smooth, ellipsoidal shape caused by water erosion.

Identified on: 4/14/2026

Mode: Standard