sedimentary
Chert (Flint variety)
Chert (Cryptocrystalline variety of Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: Dark grey to black with waxy/dull luster; Crystal Structure: Cryptocrystalline; Cleavage: None (has pronounced conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Dark grey to black with waxy/dull luster
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Physical properties
Hardness: 7 on Mohs scale; Color: Dark grey to black with waxy/dull luster; Crystal Structure: Cryptocrystalline; Cleavage: None (has pronounced conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the seafloor or by the replacement of carbonate minerals in limestone or chalk beds. Often Cretaceous to Jurassic in age.
Uses & applications
Historically used for stone tools (knapping for spear/arrowheads) and fire-starting. Currently used in road construction as aggregate and occasionally as a low-cost gemstone or lapidary material.
Geological facts
Flint was one of the most important minerals for early human development because it breaks with razor-sharp edges and creates sparks when struck against steel or iron pyrite.
Field identification & locations
Identify by its 'glassy' conchoidal fracture (curved surfaces), extreme hardness, and sharp edges. Common in limestone deposits, riverbeds, and chalk cliffs globally.
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