sedimentary
Chert (Jasper variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown with tan patches; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture)
- Hardness
- 6
- Color
- Reddish-brown with tan patches
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown with tan patches; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture)
Formation & geological history
Formed from the accumulation of silica-rich microfossils like radiolarians or diatoms on the ocean floor, followed by compaction and diagenesis. Common in sedimentary basins worldwide.
Uses & applications
Historically used for making stone tools due to sharp fracture edges; currently used for landscaping, tumbled stones, and semi-precious jewelry.
Geological facts
Chert is so durable it often remains in riverbeds long after surrounding rocks have eroded away. Red varieties colored by iron oxides are specifically called jasper.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in gravel pits and stream beds.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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