sedimentary

River Pebble (Greywacke or Siltstone)

Clastic Sedimentary Rock

AI-generated identification · may be incorrect

Hardness: 3-6 (Mohs); Color: Uniform matte grey; Luster: Dull to earthy; Texture: Fine-grained to microcrystalline; Shape: Smooth, rounded/ellipsoidal due to fluvial erosion.

Hardness: 3-6 (Mohs)
Color: Uniform matte grey
Luster: Dull to earthy

Identified June 1, 2026More sedimentary →

Explore River Pebble (Greywacke or Siltstone) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 3-6 (Mohs); Color: Uniform matte grey; Luster: Dull to earthy; Texture: Fine-grained to microcrystalline; Shape: Smooth, rounded/ellipsoidal due to fluvial erosion.

Formation & geological history

Formed from the lithification of silt and clay-sized particles in ancient marine or lacustrine environments. This specific specimen has been shaped by long-term water transport in a river or coastal environment, which ground down its sharp edges.

Uses & applications

Common pebbles are used in landscaping, construction (as aggregate), and sometimes for stone painting or decorative garden features.

Geological facts

River rocks are geological time capsules that often travel hundreds of miles from their source outcrop. Their smooth texture is a result of millions of collisions with other stones in moving water.

Field identification & locations

Identify in the field by its smooth, water-worn surface and lack of visible large crystals. Most common in riverbeds, glacial deposits, and beaches. Collectors often use them for 'worry stones'.