igneous/sedimentary
River Cobble (likely Basalt or Chert)
Siliceous River Rock / Microcrystalline Quartz / Mafic Igneous
Hardness 6-7 (Mohs scale), colors ranging from charcoal grey to earthy brown with weathering rinds, dull to waxy luster, microcrystalline structure, smooth but pitted surface texture due to river erosion.
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Physical properties
Hardness 6-7 (Mohs scale), colors ranging from charcoal grey to earthy brown with weathering rinds, dull to waxy luster, microcrystalline structure, smooth but pitted surface texture due to river erosion.
Formation & geological history
Formed through volcanic activity (if basalt) or silica precipitation (if chert), subsequently weathered and polished by fluvial (river) action over thousands of years.
Uses & applications
Commonly used as decorative landscaping stone, in construction aggregates, or as bedding for drainage. Well-worn specimens are often used as 'worry stones' or for aquarium decor.
Geological facts
River rocks are 'geological travelers'; they often originate in high mountain ranges and move hundreds of miles, becoming smaller and smoother as they tumble against other rocks in the currents.
Field identification & locations
Identify by its rounded, water-worn edges and extreme hardness (cannot be scratched by a steel knife). Commonly found in dry creek beds, riverbanks, and glacial moraines.
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