sedimentary
Chert (Flint Pebbles)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, black, or tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6
- Hardness
- 7 on Mohs scale
- Color
- Gray, black, or tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, black, or tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6
Formation & geological history
Formed as nodules in limestone or chalk through the accumulation of silica-rich organic remains (like sponges or diatoms) or chemical precipitation. Primarily found in Phanerozoic era strata.
Uses & applications
Historically used for making stone tools and starting fires; currently used as construction aggregate, road surfacing, and occasionally as a semi-precious gemstone.
Geological facts
Chert is so hard it cannot be scratched by a steel knife. It was one of the first minerals extensively traded by early humans for tool and weapon production.
Field identification & locations
Identify in the field by its smooth, waxy feel and distinct shell-like (conchoidal) fractures. Often found in riverbeds or weathered out of carbonate rock formations.
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