sedimentary

Chert / Flint

Microcrystalline quartz (SiO2)

Hardness of 7 on the Mohs scale; colors vary from white, grey, brown to black; waxy to vitreous luster; cryptocrystalline structure; conchoidal fracture with very sharp edges; specific gravity around 2.60 to 2.65.

Identified August 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the chemical replacement of limestone or chalk by silica-rich solutions in deep marine environments, or through the accumulation of siliceous skeletal remains of microscopic organisms like radiolarians and diatoms over millions of years.

Uses & applications

Historically used extensively by prehistoric humans for making stone tools, arrowheads, and blades due to its conchoidal fracture. Today used in decorative landscaping, road aggregates, and occasionally in abrasives.

Geological facts

Because of its ability to produce a bright spark when struck against steel, chert (specifically the darker variety known as flint) was historically vital for starting fires and for use in flintlock firearms.

Field identification & locations

Can be identified in the field by its distinct waxy luster, smooth conchoidal fracture surfaces, and extreme hardness which allows it to scratch glass easily. Found globally within sedimentary rock formations, especially chalk and limestone beds.