sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Tan, brown, or buff; Luster: Waxy to dull; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Tan, brown, or buff
Luster: Waxy to dull

Identified July 20, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Tan, brown, or buff; Luster: Waxy to dull; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed in marine environments through the precipitation of silica or the accumulation of siliceous organism remains (like diatoms or sponges) within limestone or chalk beds.

Uses & applications

Historically used for stone tools and starting fires; currently used as construction aggregate, road base, and by flintknapping hobbyists.

Geological facts

Because it breaks with a conchoidal fracture, it creates edges sharper than a surgeon's scalpel. It has been used by humans for hundreds of thousands of years for tools.

Field identification & locations

Identify by its waxy texture, extreme hardness (scratches glass), and shell-like (conchoidal) curved fractures. Commonly found in riverbeds or as nodules within limestone outcrops.