sedimentary

Chert (Flint)

Chert (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Gray, brown, or black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific gravity: 2.5-2.8

Hardness: 6
Color: Gray, brown, or black
Luster: Waxy to dull

Identified June 29, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs scale); Color: Gray, brown, or black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific gravity: 2.5-2.8

Formation & geological history

Formed through the accumulation of silica-rich microfossils like diatoms or radiolarians, or via chemical precipitation from silica-saturated fluids. Occurs primarily in marine environments within carbonate rocks.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used in road aggregates, landscaping, and sometimes as a gemstones when colorful.

Geological facts

Chert was one of the most important materials for early humans because its conchoidal fracture creates extremely sharp edges for hunting and processing food.

Field identification & locations

Identify by its waxy appearance, extreme hardness (it will scratch glass), and the shell-like (conchoidal) curves where it is broken. Commonly found in limestone and chalk deposits.