igneous or sedimentary

River Stone (Basalt or Siltstone)

Mafic Volcanic Rock or Clastic Sedimentary Rock

AI-generated identification · may be incorrect

Mohs hardness: 5-7; Color: Dark grey to charcoal; Luster: Dull/Matte; Structure: Fine-grained, microcrystalline, or clastic; Cleavage: None/unpredictable; Texture: Smooth, water-worn (rounded)

Hardness: 5-7
Color: Dark grey to charcoal
Luster: Dull/Matte

Identified June 2, 2026More igneous or sedimentary →

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Physical properties

Mohs hardness: 5-7; Color: Dark grey to charcoal; Luster: Dull/Matte; Structure: Fine-grained, microcrystalline, or clastic; Cleavage: None/unpredictable; Texture: Smooth, water-worn (rounded)

Formation & geological history

Formed via volcanic cooling or sediment lithification, then shaped by fluvial (river) erosion over thousands of years as the rock was transported downstream.

Uses & applications

Landscaping, decorative garden mulch, aquarium substrate, construction aggregate, and occasionally used as worrying stones or in rock tumbling hobbies.

Geological facts

River stones are often found far from their geological source, carrying the history of a drainage basin within a single pebble. Their roundness is a direct measure of how long they have been tumbled by water.

Field identification & locations

Identified by its extremely smooth, rounded edges and lack of sharp corners. Common in stream beds, beaches, and glacial deposits globally. Look for a dense, heavy feel for its size.