River Cobble (likely Basalt or Rhyolite)

Physical Properties

Hardness: 5-7 Mohs; Color: Dark grey, brownish, with surface pitting; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.6-3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich or silica-rich lava at the earth's surface. This specific specimen has been significantly weathered and rounded by water transport (fluvial processes) in a river or stream bed.

Uses & Applications

Common river stones are used for landscaping, construction aggregate, road base, and decorative garden features. Smooth specimens are sometimes used for hot stone massage.

Geological Facts

The rounded shape indicates a long journey through water, where constant tumbling and abrasion against other rocks smooth out the sharp edges of the original volcanic outcrop.

Field Identification & Locations

Identify in the field by its water-worn, rounded shape and fine-grained texture. It lacks visible large crystals, suggesting it cooled quickly. Found globally in riverbeds and glacial deposits.

Identified on: 4/15/2026

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