sedimentary
Chert Nodule
Microcrystalline Quartz (SiO2)
Hardness: 7.0 Mohs. Color: Gray, tan, white, and dark brown bands. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Conchoidal fracture.
- Hardness
- 7
- Color
- Gray, tan, white, and dark brown bands
- Luster
- Waxy to dull
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Physical properties
Hardness: 7.0 Mohs. Color: Gray, tan, white, and dark brown bands. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Conchoidal fracture.
Formation & geological history
Formed as nodules or concretions in limestone or chalk beds through the replacement of calcium carbonate by silica-rich fluids. Often Cretaceous to Paleozoic in age.
Uses & applications
Abrasives, historical tools (arrowheads/knapping), and road aggregate. Specimens with unique patterns are collected as curiosities.
Geological facts
Chert was one of the most important materials for early humans because its conchoidal fracture creates edges sharper than a steel scalpel.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and waxy appearance on broken surfaces. Often shows concentric 'cortex' growth rings.
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