sedimentary
Chert (and Flint)
Silicon Dioxide (SiO2)
Hardness: 7 Mohs; Color: Grey, blue-grey, white; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 Mohs
- Color
- Grey, blue-grey, white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs; Color: Grey, blue-grey, white; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in sedimentary basins. Most commonly found in limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons due to sharp edges; currently used as a flux in glassmaking, construction aggregate, and for decorative lapidary work.
Geological facts
Chert was one of the most important materials in human prehistory for crafting arrowheads and spear tips because its conchoidal fracture produces edges sharper than a surgical scalpel.
Field identification & locations
Identify by its extremely sharp, curved edges when broken and the fact that it cannot be scratched by a steel knife. It will also spark when struck against steel. Found globally in nodules or layered beds within carbonate rocks.
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