sedimentary

Chert (Flint)

Cryptocrystalline quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Brown, tan, or rusty (due to iron oxides); Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture).

Hardness: 6
Color: Brown, tan, or rusty (due to iron oxides)
Luster: Waxy to dull

Identified June 19, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 on Mohs scale; Color: Brown, tan, or rusty (due to iron oxides); Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture).

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from groundwater within limestone or chalk beds. Often Cretaceous to Jurassic in age but found across all geological eras.

Uses & applications

Used historically for stone tools and weapons (arrowheads), as a flint-starter for fires, and in modern road construction or as a gemstone when colorfully patterned.

Geological facts

Chert was one of the first materials used by early humans to create sharp-edged tools. It is so tough that it was used to produce sparks in early firearms like flintlocks.

Field identification & locations

Identify by its 'shell-like' conchoidal fracture, its ability to scratch glass, and its smooth, waxy texture. Commonly found in riverbeds or as nodules within limestone outcrops.