sedimentary
Chert (Jasper/Chalcedony)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (cannot be scratched by steel), Color: Tan to dark brown with banding, Luster: Waxy to vitreous, Structure: Cryptocrystalline, Fracture: Conchoidal
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (cannot be scratched by steel), Color: Tan to dark brown with banding, Luster: Waxy to vitreous, Structure: Cryptocrystalline, Fracture: Conchoidal
Formation & geological history
Formed via the accumulation of silica-rich microfossils (like radiolarians) on the ocean floor, or through the replacement of limestone by silica-rich fluids. Can be billions of years old.
Uses & applications
Historically used for stone tools (knapping) due to its sharp edges. Today used as a decorative river rock, for aquarium gravel, and occasionally as minor gemstone material.
Geological facts
Chert was the primary material for spear tips and arrowheads in prehistoric times because it breaks with a conchoidal fracture, creating razor-sharp edges.
Field identification & locations
Identified by its extreme hardness, waxy texture, and shell-like (conchoidal) fracture patterns. Frequently found as pebbles and cobbles in riverbeds or on beaches.
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