sedimentary
Chert (Black Chert/Flint)
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Black to dark gray; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.7
- Hardness
- 7 on Mohs scale
- Color
- Black to dark gray
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Black to dark gray; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.7
Formation & geological history
Formed from the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or through chemical precipitation from silica-rich fluids within limestone or chalk beds. It can date back to any geological era where marine sediments were present.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to its sharp edges. Modern uses include road surfacing (as seen in the background gravel), construction aggregate, and occasionally as a low-cost gemstone or 'worry stone' when polished.
Geological facts
Black chert is often called 'flint' when found in chalk or marly limestone. It was the primary material for the Stone Age technology because it breaks into predictably sharp, glass-like edges. It is also famous for its ability to produce sparks when struck against steel.
Field identification & locations
Identify in the field by its very smooth, curved 'conchoidal' fractures and its ability to scratch glass. It is commonly found in gravel pits, riverbeds, and as nodules within limestone outcrops. Collectors look for pieces with unique patterns or historical tool shapes.
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