sedimentary
Chert (Black Chert/Flint)
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7
- Hardness
- 7 on Mohs scale
- Color
- Dark grey to black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater. Often found as nodules in limestone or chalk beds. Geological age varies from Precambrian to Cenozoic.
Uses & applications
Used historically for stone tools and weapons (arrowheads). Modern uses include road surfacing, architectural stone, and as a starting material for silicon products. Collectors value high-quality nodules for knapping or lapidary work.
Geological facts
Black chert is often specifically called flint if found in chalk or marl. It was one of the first materials traded over long distances by prehistoric humans due to its ability to hold a razor-sharp edge.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by steel) and distinctive conchoidal (shell-like) fracture patterns. Found globally, especially in regions with extensive limestone deposits like the Midwestern US, UK, and France.
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