sedimentary

Chert (Flint)

Chert (Silicon Dioxide, SiO2)

Hardness: 7 on Mohs scale. Color: Brown, tan, and translucent grey. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).

Hardness: 7 on Mohs scale
Color: Brown, tan, and translucent grey
Luster: Waxy to dull

Identified July 6, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale. Color: Brown, tan, and translucent grey. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).

Formation & geological history

Formed as nodules in limestone or chalk through the accumulation of silica-rich organic remains (like sponge spicules or radiolaria) that dissolve and re-precipitate. Often found in ancient marine sedimentary sequences.

Uses & applications

Historically used for making stone tools (knapping) and starting fires. Today used as a construction aggregate, for landscaping, and by mineral collectors.

Geological facts

Flint is a variety of chert that was essential to human survival for millennia due to its ability to hold a sharp edge and create sparks when struck against steel.

Field identification & locations

Identify in the field by its waxy texture, smooth conchoidal (shell-like) fracture patterns, and the fact that it is hard enough to scratch glass. Commonly found in riverbeds and limestone outcrops.