sedimentary

Chert

Microcrystalline quartz (SiO2)

Hardness of 7 on the Mohs scale; varies in color from white, gray, brown, to black; waxy to vitreous luster; cryptocrystalline or microcrystalline crystal structure; conchoidal fracture with no true cleavage; specific gravity around 2.60 to 2.65.

Identified August 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the chemical precipitation of silica from aqueous solutions, or the accumulation of siliceous skeletal remains of marine organisms like diatoms and radiolarians, typically within limestone or chalk formations over millions of years.

Uses & applications

Historically used by early humans for making stone tools such as arrowheads, spear points, and blades due to its conchoidal fracture. Also used historically for striking fire, and modern uses include road aggregate and abrasive materials.

Geological facts

Chert is chemically identical to quartz, opal, and chalcedony. Flint is a well-known variety of chert that occurs specifically in chalk or marly limestone.

Field identification & locations

Identified in the field by its characteristic conchoidal fracture, sharp edges, waxy luster, and high hardness which allows it to scratch glass. Commonly found within sedimentary strata worldwide.