sedimentary

Chert (Flint Nodules)

Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Dark grey to black interior with a white/tan chalky cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic structure); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Dark grey to black interior with a white/tan chalky cortex
Luster: Waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains or chemical precipitation within limestone or chalk beds. Often associated with Cretaceous or Paleozoic marine deposits.

Uses & applications

Historically used for stone tools and weapons due to sharp edges; modernly used as road aggregate, in glass making, and by collectors for flint-knapping.

Geological facts

Flint was one of the most important resources for prehistoric humans. When struck against steel, it creates sparks, which made it the primary ignition source for flintlock firearms.

Field identification & locations

Identify by its 'rind' (cortex) on the outside and the glassy, curved (conchoidal) fracture surfaces on the inside. Found globally in chalk and limestone outcrops, especially in the UK and USA.

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Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Clastic Sedimentary Rock Grain (SiO2 dominated)

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Sandstone

Arenite (primarily SiO2)

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