sedimentary
Chert (varieties include Flint and Jasper)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs); Color: Grey, brown, tan, or reddish-brown due to iron impurities; Luster: Waxy to dull; Structure: Cryptocrystalline (crystals too small to see with a microscope); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7
- Hardness
- 7 (Mohs)
- Color
- Grey, brown, tan, or reddish-brown due to iron impurities
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs); Color: Grey, brown, tan, or reddish-brown due to iron impurities; Luster: Waxy to dull; Structure: Cryptocrystalline (crystals too small to see with a microscope); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7
Formation & geological history
Formed through the accumulation of silica-rich marine organisms (like diatoms or radiolarians) or the replacement of limestone by silica-rich fluids. Can be billions of years old, often found in Precambrian through Cenozoic formations.
Uses & applications
Historically used for stone tools (knapping) and starting fires. Modern use in abrasives, construction aggregate, and occasionally semi-precious jewelry (Jasper varieties).
Geological facts
Chert is so hard it can scratch steel. It was the most important material for early human tools like arrowheads and axes because it breaks with sharp, glass-like edges called conchoidal fractures.
Field identification & locations
Identify by its waxy texture, inability to be scratched by a steel knife, and smooth, curved (shell-like) fracture patterns. Found worldwide in sedimentary rock beds, riverbeds, and as nodules in limestone.
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