sedimentary

Chert (and Flint)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Grey, black, or tan interior with a weathered brown cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Grey, black, or tan interior with a weathered brown cortex
Luster: Waxy to vitreous

Identified June 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the ocean floor, or from the chemical precipitation of silica in groundwater within limestone or chalk beds.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) and starting fires. Modernly used as road aggregate, architectural stone, and occasionally in jewelry as lapidary material.

Geological facts

Flint is a variety of chert that occurs in chalk or marly limestone. It was the primary material for the Stone Age technology due to its ability to hold a sharp edge when fractured.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture pattern and its extreme hardness (it will scratch glass). Often found as nodules within limestone outcrops or as rounded river cobbles.