igneous and sedimentary

River Pebbles (Basalt and Graywacke/Limestone)

Mafic Volcanic Rock / Impure Sandstone

Hardness: 5-7 (Mohs scale); Color: Dark grey to charcoal black (top) and medium grey with calcite veining (bottom); Luster: Dull/Matte; Structure: Fine-grained/Aphanitic; Shape: Well-rounded/discoidal due to water erosion.

Hardness: 5-7 (Mohs scale)
Luster: Dull/Matte

Identified July 20, 2026More igneous and sedimentary →

Explore River Pebbles (Basalt and Graywacke/Limestone) in the encyclopedia →

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Physical properties

Formation & geological history

Formed through volcanic cooling (top) or sedimentary deposition (bottom) followed by long-term mechanical weathering in a high-energy aquatic environment such as a riverbed or coastal beach.

Uses & applications

Landscaping, decorative garden mulch, aquarium substrate, massage stones (basalt), and architectural pebble mosaic.

Geological facts

The smooth, rounded shape is a result of a process called 'abrasion,' where rocks collide with each other in water, wearing down sharp edges over thousands of years.

Field identification & locations

Identify by their smooth, 'soft' texture and lack of sharp edges; common in river basins and glacial deposits. The white lines on the bottom pebble are likely secondary calcite or quartz veins filling ancient fractures.

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