sedimentary
Chert (Flint Nodules)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale; Color: Variable (tan, grey, brown, or white); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on the Mohs scale
- Color
- Variable (tan, grey, brown, or white)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on the Mohs scale; Color: Variable (tan, grey, brown, or white); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed as nodules or concretions in limestone or chalk through a replacement process where silica-rich fluids precipitate in cavities within the rock matrix. This often happens in marine environments over millions of years.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) and for sparking fires (flint and steel). Modern uses include road surfacing material and as a decorative stone in landscaping.
Geological facts
Chert is so durable that it can survive long after the surrounding limestone has eroded away. Historically, flint was the primary material used for the ignition mechanism in flintlock firearms.
Field identification & locations
Identify in the field by its very hard surface (can scratch glass), smooth waxy texture when broken, and characteristic curved, shell-like (conchoidal) fractures. Often found in riverbeds or chalk cliffs.
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