sedimentary

Chert

Microcrystalline quartz (SiO2)

Hardness of 7 on the Mohs scale, colors vary widely including brown, gray, white, and yellowish-brown, dull to waxy luster, conchoidal fracture with sharp edges, no cleavage, specific gravity around 2.60-2.65.

Identified August 7, 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 such as radiolarians and diatoms, typically during the Mesozoic and Paleozoic eras.

Uses & applications

Historically used by ancient humans for making stone tools such as arrowheads, spear points, and knives due to its conchoidal fracture. Also used historically in flintlock firearms and occasionally as decorative gravel or landscaping material.

Geological facts

Chert is a durable and weather-resistant rock that often forms nodules within limestone or chalk formations. Varieties that are dark or black are commonly referred to as flint.

Field identification & locations

Can be identified in the field by its distinct conchoidal fracture, extremely high hardness that can scratch glass, and lack of effervescence when tested with dilute hydrochloric acid. Common worldwide in sedimentary deposits.