sedimentary

Chert (Flint)

Chert (SiO2)

Hardness: 7 (Mohs scale), Color: Brown, tan, white, and earthy tones, Luster: Waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline quartz, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7

Identified July 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or through the chemical precipitation of silica in marine or groundwater environments. Commonly found as nodules in limestone or chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; modern uses include road construction, landscaping, and occasionally as a source for silica in industrial processes.

Geological facts

Chert is so hard it can scratch steel. When struck against steel, it can produce sparks, which is why it was historically vital for flintlock firearms and fire-starting. The white patches on the surface are likely a weathered 'rind' or cortex of dehydrated silica.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to easily scratch glass. It is commonly found in gravel pits, riverbeds, or embedded in sedimentary layers. Collectors look for unique colors or fossil inclusions.