sedimentary

River Pebble (Greywacke / Sandstone Pebble)

Indurated Sandstone / Greywacke Pebble

Hardness: 6-7 on Mohs scale; Color: Dark brownish-grey to dull olive-grey with small grainy flecks; Luster: Dull to earthy; Texture: Fine-grained, sub-rounded to well-rounded river pebble; Specific gravity: ~2.6-2.7

Hardness: 6-7 on Mohs scale
Luster: Dull to earthy

Identified July 26, 2026More sedimentary →

Explore River Pebble (Greywacke / Sandstone Pebble) in the encyclopedia →

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Physical properties

Formation & geological history

Formed initially from sedimentary deposition of sand, silt, and quartz grains in marine or fluvial environments, followed by deep burial and compaction. Subsequent erosion and transportation by river water naturally shaped and rounded the pebble over thousands of years.

Uses & applications

Commonly used in landscaping, river rock decor, aggregate for concrete, aquarium substrate, and pocket stones.

Geological facts

River pebbles get their characteristic smooth, rounded shapes from the process of attrition, where stones roll along riverbeds, grinding against one another over extended periods.

Field identification & locations

Field identification: Look for smooth, rounded edges, a grainy sedimentary texture under magnification, and a dull surface finish when dry. Commonly found along stream beds, riverbanks, ocean beaches, and glacial gravel deposits worldwide.

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