sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Grey with tan/buff weathered cortex; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Grey with tan/buff weathered cortex
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Grey with tan/buff weathered cortex; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the chemical precipitation of silica-rich fluids within sedimentary rock layers, often limestone or chalk, frequently replacing organic material or filling voids. Most are Mesozoic or Cenozoic in age.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to its sharp edges; modern uses include construction aggregate, road surfacing, and as a collector specimen when featuring unique shapes.
Geological facts
Chert is so durable that it often remains in creek beds long after the surrounding limestone has eroded away. Historically, certain types like flint were essential for starting fires and making firearms work (flintlocks).
Field identification & locations
Identify in the field by its waxy texture and shell-like (conchoidal) fractures. It will easily scratch glass. Commonly found in limestone outcrops, riverbeds, and gravel pits globally.
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