Basalt Pebble

Physical Properties

Hardness: 6-7 on Mohs scale; Color: dark grey to black with possible brown iron-oxide staining; Luster: dull to sub-glassy; Crystal structure: fine-grained (aphanitic) sometimes with tiny vesicles (vesicular); Specific gravity: 2.8 - 3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. This specific specimen has been rounded by fluvial (river) or coastal (wave) erosion.

Uses & Applications

Used widely in construction as crushed stone, road base, and railroad ballast. Smooth basalt pebbles are popular in landscaping and decorative aquariums.

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 (relative to size), and very fine-grained texture. It will not scratch with a copper penny but can be scratched by quartz.

Identified on: 4/15/2026

Mode: Standard