sedimentary
Chert Nodules or Concretion
Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Brown, tan, and cream; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); High specific gravity relative to porous rocks.
- Hardness
- 7 on Mohs scale
- Color
- Brown, tan, and cream
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Brown, tan, and cream; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); High specific gravity relative to porous rocks.
Formation & geological history
Formed via chemical precipitation of silica-rich fluids within sedimentary rock layers, often replacing organic material or filling voids in limestone or chalk during diagenesis.
Uses & applications
Used historically for stone tools (arrowheads/scrapers) due to its sharp edges when broken; currently used in decorative landscaping and as a collector's curiosity.
Geological facts
Chert is so durable that it often remains behind as 'lag' after the surrounding softer limestone has eroded away. It is often confused with fossilized bone due to its organic shapes.
Field identification & locations
Identify by its hardness (it will scratch glass) and its smooth, waxy texture compared to the surrounding rock. Common in limestone outcrops and riverbeds.
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