sedimentary
Chert (specifically Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Brown, grey, or black; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Brown, grey, or black
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Brown, grey, or black; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds.
Uses & applications
Historically used for stone tools (knapping) and starting fires. Today used in construction aggregate, road metal, and as a semi-precious gemstone in jewelry when it displays colorful patterns.
Geological facts
Flint was one of the most important minerals to early humans for making arrowheads and spear points due to its sharp 'conchoidal' edges. It spark when struck against steel.
Field identification & locations
Identify by its extremely smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Commonly found in riverbeds or coastal chalk cliffs.
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