sedimentary
Chert (Flint)
Chert (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, brown, or tan with a weathered yellowish-brown exterior; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None, exhibits conchoidal fracture.
- Hardness
- 7 (Mohs scale)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, brown, or tan with a weathered yellowish-brown exterior; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None, exhibits conchoidal fracture.
Formation & geological history
Formed in marine or freshwater environments as a chemical precipitate or through the replacement of carbonaceous material. Commonly found as nodules or layers within limestone or chalk beds.
Uses & applications
Historically used for stone tools and fire-starting (flint); modernly used in road construction, stone aggregates, and occasionally as semi-precious gemstones when colorful (jasper).
Geological facts
Chert is extremely sharp when broken, which is why ancient civilizations used it for arrowheads and scrapers. Its ability to produce sparks when struck against steel made it essential for flintlock firearms.
Field identification & locations
Identify by its conchoidal (shell-like) fracture patterns and ability to scratch glass. It often has a chalky white or brown weathered 'cortex' or rind on the outside. Common in sedimentary basins worldwide.
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