Basalt (River Rock Cobble)

Physical Properties

Hardness: 6-7 (Mohs); Color: Dark grey to charcoal black; Luster: Dull to sub-metallic when wet; Crystal structure: Aphanitic (fine-grained); Dense and heavy.

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 stream environment.

Uses & Applications

Commonly used in construction as aggregate, for road base, in landscaping, and sometimes for hot stone massage therapy due to its heat retention.

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 in the field by its dark color, heavy weight for its size, and lack of visible large crystals. Rounded cobbles like this are common in ancient and modern riverbeds.

Identified on: 5/1/2026

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