sedimentary
Banded Chert (River Cobble)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: tan, brown, and cream bands, Luster: waxy to vitreous when wet, Structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: tan, brown, and cream bands, Luster: waxy to vitreous when wet, Structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the chemical precipitation of silica-rich fluids or the accumulation of siliceous remains of organisms like diatoms and radiolarians. This specimen shows water-worn rounding from river transport.
Uses & applications
Used historically for stone tools and arrowheads due to conchoidal fracture. Currently used as decorative landscaping stones, aquarium gravel, or for rock tumbling.
Geological facts
Banded chert is often associated with ancient seafloors and can sometimes contain microfossils. Because of its extreme hardness, it survives long journeys in river systems while softer rocks erode away.
Field identification & locations
Identify in the field by its waxy texture, bands that do not rub off, and its ability to scratch glass. It is commonly found in gravel bars of rivers and ancient sedimentary basins.
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