sedimentary

Chert

Microcrystalline quartz (SiO2)

Hardness of 7 on the Mohs scale, typically brown, tan, or gray in color, waxy to vitreous luster, amorphous or cryptocrystalline crystal structure, conchoidal fracture with no true cleavage, specific gravity of 2.60 to 2.65.

Identified August 7, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed via the chemical precipitation of silica from aqueous solutions, often derived from the skeletal remains of microscopic marine organisms like radiolarians or diatoms accumulating on the ocean floor, over geological ages ranging from Paleozoic to Cenozoic.

Uses & applications

Historically used by early humans for making stone tools such as arrowheads and spear points due to its sharp conchoidal fracture. Also used historically for striking fire.

Geological facts

Chert that is dark-colored or black is often referred to as flint. It is extremely durable and weather-resistant, which is why it often survives in the geological record long after surrounding limestone has eroded.

Field identification & locations

Can be identified in the field by its extreme hardness (scratches glass), waxy luster, and distinctive conchoidal fracture resembling broken glass. Commonly found as nodules within limestone or chalk formations worldwide.