sedimentary
Chert (Jasper variety)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs). Color: Brownish-yellow to tan with opaque luster. Crystal structure: Cryptocrystalline (microscopic quartz). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.6-2.7.
- Hardness
- 6
- Color
- Brownish-yellow to tan with opaque luster
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Physical properties
Hardness: 6.5-7 (Mohs). Color: Brownish-yellow to tan with opaque luster. Crystal structure: Cryptocrystalline (microscopic quartz). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.6-2.7.
Formation & geological history
Formed through the precipitation of silica-rich fluids within sedimentary rock layers like limestone or through the accumulation of siliceous organisms. Often Mesozoic to Cenozoic in age depending on the local basin.
Uses & applications
Used historically for stone tools due to sharp edges when fractured. Modern uses include decorative landscaping, tumbling for jewelry/beads, and as an industrial abrasive.
Geological facts
Chert is so durable that it often survives as smooth river pebbles long after the surrounding host rock has eroded away. Certain colorful varieties are highly sought after by lapidaries.
Field identification & locations
Identify by its extreme hardness (will scratch glass/steel) and smooth, waxy feel when wet. Commonly found in riverbeds, gravel pits, and coastal regions as rounded cobbles.
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