sedimentary
Chert (Quartzite-rich River Pebble)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: tan to grayish-brown, Luster: waxy to vitreous, Crystal Structure: Microcrystalline/cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: tan to grayish-brown, Luster: waxy to vitreous, Crystal Structure: Microcrystalline/cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed as a chemical precipitate from silica-rich fluids or the accumulation of siliceous microfossils like diatoms or radiolarians. This specimen is water-worn, indicating transport through a river or stream system over thousands of years.
Uses & applications
Historically used for stone tools (arrowheads) due to its sharp fracture edges. Today used in construction as river rock, aggregate for concrete, and occasionally as decorative landscaping stone.
Geological facts
Chert is so hard it can scratch glass and steel. This specific specimen’s rounded shape is a result of fluvial erosion, a process that physically weathers the rock as it tumbles along riverbeds.
Field identification & locations
Identify in the field by its hardness (will scratch a pocket knife blade) and its smooth, rounded 'potato' look when found in riverbeds. Common globally in alluvial deposits and ancient seafloors.
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