sedimentary

River Pebble (Siltstone/Mudstone)

Siltstone

Hardness: 3-4 (Mohs scale); Color: Tan to grayish-brown; Luster: Dull/Earthy; Structure: Fine-grained clastic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Hardness: 3-4 (Mohs scale)
Color: Tan to grayish-brown
Luster: Dull/Earthy

Identified July 20, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs scale); Color: Tan to grayish-brown; Luster: Dull/Earthy; Structure: Fine-grained clastic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Formation & geological history

Formed from the lithification of silt-sized particles deposited in low-energy environments like river beds, floodplains, or lagoons. The rounded shape is the result of mechanical weathering and water erosion (abrasion) over long periods.

Uses & applications

Commonly used in landscaping, as aggregate in construction, or as decorative stones. Occasionally used by hobbyists for rock painting or tumbling.

Geological facts

This specific shape, known as an ellipsoid, is a classic example of fluvial erosion where water current rolls the stone along a riverbed, gradually wearing down all sharp edges into a smooth surface.

Field identification & locations

Identify in the field by its smooth, water-worn texture and lack of visible crystals. Commonly found in riverbanks, stream beds, and glacial deposits. Collectors often look for unique ergonomic shapes.