sedimentary
Chert (Field Stone)
Chert (SiO2)
Hardness: 7 (Mohs); Color: Gray, tan, brown with white speckling; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs)
- Color
- Gray, tan, brown with white speckling
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs); Color: Gray, tan, brown with white speckling; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich microfossils or precipitation from hydrothermal fluids. Common in marine sedimentary environments. This specimen shows significant rounding from water erosion.
Uses & applications
Historically used for stone tools and ritual items; currently used as road aggregate, architectural stone, and occasionally for lapidary work.
Geological facts
Chert was one of the most important resources for prehistoric humans due to its ability to be knapped into sharp edges. Flint is a specific variety of dark chert.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and smooth, shell-like fracture surfaces. Often found in riverbeds, glacial tills, and outcrops of limestone or chalk.
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