sedimentary
Chert Cobble / Quartzite River Stone
Microcrystalline Quartz (SiO2) / Quartzite
Hardness 7 on the Mohs scale, earthy tan to brownish gray color, dull to waxy luster, conchoidal fracture, lacks visible crystal structure to the naked eye, non-porous.
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Physical properties
Hardness 7 on the Mohs scale, earthy tan to brownish gray color, dull to waxy luster, conchoidal fracture, lacks visible crystal structure to the naked eye, non-porous.
Formation & geological history
Formed from the accumulation of silica-rich organic remains or chemical precipitation in marine or sedimentary environments, subsequently rounded and smoothed by stream or river erosion over time.
Uses & applications
Used historically for stone tools and knapping; modern uses include construction aggregate, landscaping gravel, river rock, and lapidary tumbling.
Geological facts
Smooth river cobbles like this are shaped by millions of collisions with other rocks and sand in stream beds. Cryptocrystalline silicas such as chert were among the most essential raw materials for early toolmaking humans due to their sharp conchoidal fracture edges.
Field identification & locations
Identified in the field by its smooth rounded shape, hardness (cannot be scratched by a steel blade), dull to slightly waxy texture, and light tan/brownish hue. Extensively found along riverbeds, glacial tills, and alluvial plains worldwide.
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