sedimentary

Chert / Flint

Microcrystalline Quartz (SiO2)

Hardness of 7 on the Mohs scale, colors range from white, grey, brown to black, waxy to vitreous luster, cryptocrystalline structure, conchoidal fracture with very sharp edges, no true cleavage.

Identified August 2, 2026More sedimentary →

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Physical properties

Hardness of 7 on the Mohs scale, colors range from white, grey, brown to black, waxy to vitreous luster, cryptocrystalline structure, conchoidal fracture with very sharp edges, no true cleavage.

Formation & geological history

Forms in marine environments through the chemical replacement of calcium carbonate in limestone or chalk by silica-rich solutions, or through the accumulation of siliceous skeletal remains of microscopic organisms like radiolaria and diatoms. Geological age varies widely from Cretaceous to Paleozoic.

Uses & applications

Historically used for making stone tools, arrowheads, and striking fire. Modern uses include abrasives, decorative gravel, and lapidary work.

Geological facts

Chert that is dark grey or black is commonly referred to as flint. It was one of the earliest materials manipulated by humans for primitive technology due to its predictable conchoidal fracture.

Field identification & locations

Found worldwide within chalk and limestone formations. Identifiable by its dull chalky cortex on the outside and smooth, glassy, conchoidally fractured interior when broken.