sedimentary

Chert

Cryptocrystalline quartz (SiO2)

Hardness of 7 on the Mohs scale, waxy to vitreous luster, conchoidal fracture, various colors including grey, white, brown, or black, amorphous or microcrystalline crystal structure, specific gravity around 2.60 to 2.66.

Identified August 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the chemical precipitation of silica from aqueous solutions, often replacing carbonate rocks like limestone during diagenesis, or from the accumulation of siliceous skeletal remains of microscopic organisms such as radiolarians and diatoms.

Uses & applications

Historically used by early humans for making stone tools like arrowheads and knife blades due to its sharp conchoidal fracture. Also used historically for striking fire with steel, and occasionally as a decorative or abrasive stone.

Geological facts

Chert that occurs in nodules inside limestone or chalk formations is commonly known as flint. Because it is extremely resistant to weathering, chert nodules often survive long after the surrounding limestone has eroded away.

Field identification & locations

Can be identified in the field by its hardness (scratches glass), waxy luster, and distinctive sharp conchoidal fracture surfaces. Commonly found embedded in limestone or as pebble deposits in riverbeds.