sedimentary
Chert (Jasper/Agate variety)
Microcrystalline quartz (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Brownish-tan with darker rims. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoïdal fracture).
- Hardness
- 6
- Color
- Brownish-tan with darker rims
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Brownish-tan with darker rims. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoïdal fracture).
Formation & geological history
Formed from the accumulation of siliceous remains of organisms (like diatoms or radiolaria) or chemical precipitation from groundwater within sedimentary layers. Often billions to millions of years old.
Uses & applications
Used historically for stone tools (flintknapping); current uses include lapidary work, jewelry (tumbled stones), and as decorative landscaping gravel.
Geological facts
Chert is so durable that it often survives as smooth river pebbles long after the surrounding rock has eroded away. It was one of the first materials used by early humans to create fire and sharp tools.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, waxy feel when wet. Often found in riverbeds or gravel pits.
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