sedimentary
Chert / Flint
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, brown, or tan with a waxy luster; Crystal Structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 (Mohs scale)
- Color
- Gray, brown, or tan with a waxy luster
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, brown, or tan with a waxy luster; Crystal Structure: Hexagonal (microscopic); 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 the replacement of limestone by silica-rich fluids. Can be found in bedding or as nodules within limestone/chalk.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to sharp fracture edges and for starting fires (flint). Currently used in road construction and occasionally as landscaping stone.
Geological facts
Flint is a variety of chert. Because it breaks with a conchoidal fracture, it was the primary material used by prehistoric humans for lithic technology. It can produce sparks when struck against steel.
Field identification & locations
Look for a very fine-grained, smooth texture and distinctive curved (conchoidal) breaks that resemble glass. Often found in chalk cliffs or limestone outcrops. Collectors look for pieces with unique colors or banding.
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