sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 Mohs. Color: Grey, tan, or brown with a waxy luster. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.

Hardness: 6
Color: Grey, tan, or brown with a waxy luster

Identified July 15, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs. Color: Grey, tan, or brown with a waxy luster. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.

Formation & geological history

Formed as nodules or beds in sedimentary rocks through the precipitation of silica-rich fluids. Often contains microfossils and is commonly found in limestone or chalk deposits dating across various geological eras.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp edges; used today as decorative gravel, in road construction, and for fire-starting strikers.

Geological facts

Chert has been used by humans for millions of years. When struck against steel, it produces sparks, which led to its widespread use in flintlock firearms and early fire-making techniques.

Field identification & locations

Identify by its waxy texture, extreme hardness (it will scratch glass), and its tendency to break into sharp, curved flakes (conchoidal fracture). Common in riverbeds and limestone outcrops.