sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness 7 on Mohs scale; light grey interior with reddish-brown oxidized outer weathering rind; waxy to dull luster; microcrystalline/cryptocrystalline texture; conchoidal fracture.

Identified July 23, 2026More sedimentary →

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Physical properties

Hardness 7 on Mohs scale; light grey interior with reddish-brown oxidized outer weathering rind; waxy to dull luster; microcrystalline/cryptocrystalline texture; conchoidal fracture.

Formation & geological history

Formed by the precipitation of silica in marine environments or groundwater silica replacement, followed by chemical weathering and iron oxide stain accumulation on the outer surface over thousands of years.

Uses & applications

Historically used for knapping stone tools, arrowheads, and spark-striking flintlocks. Currently used in road construction, aggregates, and as a popular specimen for rockhounding.

Geological facts

Chert flakes produce extremely sharp edges, often sharper than steel surgical scalpels. The distinct reddish-brown crust is caused by iron oxidation from exposure to air and water.

Field identification & locations

Identified by its microcrystalline texture, distinct conchoidal fracture, and extreme hardness (scratches glass). Commonly found in limestone formations, gravel beds, and river basins worldwide.