sedimentary
Greywacke Siltstone Pebble / Concretionary River Pebble
Siltstone / Argillite (Fine-grained Sedimentary Rock)
Hardness: 3-5 Mohs. Color: Medium grayish-green to dark slate gray. Luster: Dull, earthy, or matte. Texture: Smooth, water-worn fine-grained matrix with raised differential erosion rings or concretionary banding.
- Hardness
- 3-5 Mohs
- Color
- Medium grayish-green to dark slate gray
- Luster
- Dull, earthy, or matte
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Physical properties
Hardness: 3-5 Mohs. Color: Medium grayish-green to dark slate gray. Luster: Dull, earthy, or matte. Texture: Smooth, water-worn fine-grained matrix with raised differential erosion rings or concretionary banding.
Formation & geological history
Formed from the compaction and lithification of mud, silt, and fine sand deposits in marine or fluvial environments. Subsequent natural weathering and mechanical abrasion in riverbeds or beaches shaped its rounded form and exposed internal compositional layers.
Uses & applications
Used primarily as decorative river stones, landscaping materials, or pocket stones/curiosities due to its unusual, tactile, weathered shape.
Geological facts
The distinct raised ridges and ring patterns are caused by differential weathering, where harder mineral-enriched layers (often calcium carbonate or iron oxide concentrations) resist water erosion better than the softer surrounding matrix.
Field identification & locations
Commonly found in river beds, glacial till deposits, and gravel bars near mountainous regions. Identified by its smooth, water-rounded surface, fine grain size, dull gray-green tone, and raised erosion bands.
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