sedimentary
Chert (Flint)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 Mohs; Color: Brown, grey, black; Luster: Waxy to vitreous; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 Mohs
- Color
- Brown, grey, black
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs; Color: Brown, grey, black; Luster: Waxy to vitreous; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed primarily in marine environments where microcrystalline silica precipitates from water or replaces organic material (like limestone or chalk) during diagenesis.
Uses & applications
Historically used for stone tools and starting fires; currently used as road aggregate, in glass making, and by flintknapping hobbyists.
Geological facts
Flint is a variety of chert. When struck against steel, it creates sparks due to the friction oxidizing iron particles. It has been used by humans for millions of years for tools.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture, sharp edges when broken, and extreme hardness (it will scratch glass). Commonly found in sedimentary basins.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Sandstone
Arenite (primarily SiO2)
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