Basalt River Cobble

Physical Properties

Hardness: 6.0; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.87-3.00

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specific specimen has been rounded and smoothed by water transport in a river or coastal environment.

Uses & Applications

Used in construction for road base, concrete aggregate, and railroad ballast. Smooth river stones are popular in landscaping and decorative masonry.

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.

Field Identification & Locations

Identify by its heavy weight (density), dark color, and fine grain where individual crystals are not visible to the naked eye. Found worldwide in volcanic provinces and riverbeds.

Identified on: 5/1/2026

Mode: Standard