sedimentary

Chert (Flint)

Cryptocrystalline Quartz (SiO2)

Hardness: 7 Mohs; Color: Brown, tan, white, and translucent; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 Mohs
Color: Brown, tan, white, and translucent
Luster: Waxy to dull

Identified July 17, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Brown, tan, white, and translucent; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed in marine environments as nodules or layered deposits when silica-rich fluids precipitate within limestone or chalk beds. Often contains microfossils.

Uses & applications

Historically used for making stone tools (knapping) and sparking fire. Today used for decorative landscaping, road surface material, and occasionally semi-precious jewelry.

Geological facts

Chert has been used by humans for over 2 million years to create sharp-edged tools like arrowheads. When struck against steel, it produces hot sparks used for fire-starting.

Field identification & locations

Identified in the field by its smooth, waxy surface and sharp, curved (conchoidal) edges when broken. It cannot be scratched by a steel knife. Common in limestone outcrops.