sedimentary
Chert (Black Flint)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 Mohs scale; Color: Dark grey to black; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 Mohs scale
- Color
- Dark grey to black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs scale; Color: Dark grey to black; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed in marine environments through the precipitation of silica or the replacement of carbonate minerals by silica-rich fluids. This specimen appears water-worn or tumbled.
Uses & applications
Historically used for stone tools and starting fires; currently used as aquarium decor, in jewelry (as 'onyx' substitutes), or as industrial abrasives.
Geological facts
Black chert is often called flint. It was one of the most important minerals for early humans due to its ability to break into extremely sharp edges.
Field identification & locations
Check for conchoidal (shell-like) fractures and look for a waxy texture. It will easily scratch glass and cannot be scratched by a steel knife. Common in riverbeds and coastal areas.
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