sedimentary
Chert (White Chert/Flint)
Chert (Cryptocrystalline Silica or Microcrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: White to off-white/light gray; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- White to off-white/light gray
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: White to off-white/light gray; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica in sedimentary environments, often replacing calcium carbonate or occurring as nodules in limestone. It can also form from the accumulation of silica-rich microfossils like diatoms or radiolarians.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; used today as road aggregate, architectural stone, and occasionally for lapidary work.
Geological facts
Chert is so hard it can scratch steel. When struck against steel, it can produce sparks, leading to its historical use as a fire starter and in flintlock firearms.
Field identification & locations
Identify by its waxy luster, extreme hardness, and conchoidal (shell-like) fracture patterns. It is very common in limestone and dolomite formations globally.
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