sedimentary

Chert / Flint (with Iron Staining)

Silicon Dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 Mohs scale; Color: Grey, tan, brown with yellow/orange iron oxidation; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6

Hardness: 7 Mohs scale
Color: Grey, tan, brown with yellow/orange iron oxidation
Luster: Waxy to dull

Identified March 26, 2026More sedimentary →

Explore Chert / Flint (with Iron Staining) in the encyclopedia →

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Physical properties

Formation & geological history

Formed as nodules or beds in limestone or chalk through the precipitation of silica from groundwater. The silica often comes from biological sources like sponge spicules or radiolarians. Most findable specimens range from the Paleozoic to the Cretaceous periods.

Uses & applications

Historically used for stone tools/weapons due to sharp edges; currently used as construction aggregate, road ballast, or by hobbyists for flint knapping (arrowhead making) and fire-starting.

Geological facts

Chert was one of the first resources 'mined' by prehistoric humans. Because of its extreme hardness and way it breaks into razor-sharp edges, it was essential for survival. It is chemically identical to quartz but has a different internal structure.

Field identification & locations

Identify in the field by its conchoidal (shell-like) fracture patterns and the fact that it will easily scratch glass. Commonly found in stream beds or weathering out of limestone cliffs. Collectors should look for 'glassy' textures inside rough exterior rinds.