sedimentary
River Cobble with Algae Patina
Weathered Chert or Quartzite pebble (SiO2 dominated)
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)
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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.
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