sedimentary
Chert (Flint)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 Mohs; Color: Gray, tan, black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.65
- Hardness
- 6
- Color
- Gray, tan, black
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Gray, tan, black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.65
Formation & geological history
Formed as nodules or layered deposits in limestone/chalk through the chemical replacement of host rock or accumulation of siliceous organisms.
Uses & applications
Historical tool making (arrowheads), fire-starting, road aggregate, and occasionally used in landscaping or as semi-precious gemstones.
Geological facts
Chert has been used by humans for millions of years to create sharp-edged tools. When struck against steel, it produces sparks, earning its use in flintlock firearms.
Field identification & locations
Identify by its extreme hardness (scratches glass) and distinctive smooth, curved 'conchoidal' fractures. Often found in seafloor sedimentary sequences.
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