sedimentary

River Cobble with Algae Patina

Weathered Chert or Quartzite pebble (SiO2 dominated)

AI-generated identification · may be incorrect

Hardness: 6.5-7 on Mohs scale; Color: Tan to brown base with green biological staining; Luster: Dull/Earthy (unpolished); Crystal Structure: Cryptocrystalline; Specific Gravity: Approx 2.6

Hardness: 6
Color: Tan to brown base with green biological staining
Luster: Dull/Earthy (unpolished)

Identified July 5, 2026More sedimentary →

Explore River Cobble with Algae Patina in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 6.5-7 on Mohs scale; Color: Tan to brown base with green biological staining; Luster: Dull/Earthy (unpolished); Crystal Structure: Cryptocrystalline; Specific Gravity: Approx 2.6

Formation & geological history

Formed via the weathering and transport of larger rock masses in an aquatic environment. The smooth, rounded shape is a result of long-term fluvial erosion (river action), likely from the Holocene or Pleistocene epochs.

Uses & applications

Common river stones are used in landscaping, as aggregate in concrete construction, and for aquarium decoration.

Geological facts

The green coloration is not part of the rock itself but is a 'biofilm' of algae or lichen that develops when the stone is exposed to moisture and sunlight and then dries.

Field identification & locations

Identify by its rounded 'potato' shape and smooth surface texture compared to jagged outcrop rocks. Common in riverbeds, glacial till, and coastal areas.