sedimentary
Chert (and Flint)
Silicon Dioxide (Microcrystalline Quartz), SiO2
Hardness: 7 on Mohs scale; Color: Gray, tan, white, brown; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, white, brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, white, brown; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. Commonly found in Paleozoic and Mesozoic strata.
Uses & applications
Historically used for stone tools and weapons. Modern uses include road surfacing, decorative stone, and as a component in glass and ceramics production.
Geological facts
Chert was a vital resource for prehistoric humans because it breaks with a conchoidal fracture, creating sharp edges for arrowheads and knives. It is also famous for its ability to create sparks when struck against steel.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in nodular or layered form within limestone outcrops.
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