sedimentary

Chert (Flint)

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, brown, or gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Tan, brown, or gray
Luster: Waxy to dull

Identified May 23, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Tan, brown, or gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed by the accumulation of siliceous remains of marine organisms or through the chemical precipitation of silica in groundwater. Often found as nodules or layers within limestone or chalk beds.

Uses & applications

Historically used for making sharp tools and arrowheads. Currently used as a road surfacing material, in construction aggregate, and occasionally for lapidary work and fire-starting kits.

Geological facts

Because it breaks with a conchoidal fracture like glass, it produces incredibly sharp edges. It has been used by humans for hundreds of thousands of years to create tools and weapons.

Field identification & locations

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in stream beds or protruding from limestone outcrops.