sedimentary
Chert (Flint)
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, black, or brown; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Gray, black, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, black, or brown; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed by the accumulation of siliceous remains of marine organisms (diatoms, radiolarians) or by chemical precipitation in groundwater replacing limestone/chalk. Often found in nodules within sedimentary layers.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Currently used in road surfacing, architectural aggregate, and occasionally as semi-precious gemstones when colorful.
Geological facts
Chert has been used since the Stone Age for spearheads and arrowheads due to its ability to split into extremely sharp edges. It is one of the most durable rocks on the Earth's surface.
Field identification & locations
Identify by its waxy appearance and distinct shell-like (conchoidal) fracture pattern when broken. It cannot be scratched by a steel nail. Commonly found in riverbeds or within limestone outcrops.
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