sedimentary

Chert (Flint) with Limestone Coating

Silicon Dioxide (SiO2) with Calcium Carbonate (CaCO3) cortex

AI-generated identification · may be incorrect

Hardness: 7 (inner core), 3 (outer cortex); Color: Brownish-grey core with tan/yellow outer rinde; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture)

Hardness: 7 (inner core), 3 (outer cortex)
Color: Brownish-grey core with tan/yellow outer rinde
Luster: Waxy to dull

Identified July 11, 2026More sedimentary →

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Physical properties

Hardness: 7 (inner core), 3 (outer cortex); Color: Brownish-grey core with tan/yellow outer rinde; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture)

Formation & geological history

Formed through the replacement of limestone or chalk by silica-rich fluids. Often found as nodules within sedimentary basins, particularly from the Cretaceous period.

Uses & applications

Historically used for stone tools and starting fires; modernly used in landscaping, as aggregate, or as a lapidary material for hobbyists.

Geological facts

Flint is a variety of chert that occurs primarily in chalk and marly limestone. The outer 'skin' or cortex is often softer and more porous than the glassy interior.

Field identification & locations

Identify by its heavy weight and sharp, glass-like edges when broken (conchoidal fracture). It will scratch glass but cannot be scratched by a steel knife.