sedimentary

Chert

Microcrystalline quartz (SiO2)

Hardness: 7 on Mohs scale, Color: Brown, gray, white, or yellowish-brown, Luster: Waxy to vitreous, Crystal structure: Microcrystalline/cryptocrystalline quartz (chalcedony), Cleavage: None (exhibits conchoidal fracture), Specific gravity: 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-rich fluids or the accumulation of siliceous skeletal remains of microscopic marine organisms (such as radiolarians or diatoms) in marine environments, typically over millions of years within limestone or chalk deposits.

Uses & applications

Historically used by ancient humans for making stone tools, arrowheads, and blades due to its sharp conchoidal fracture; occasionally used for landscaping, decorative gravel, and spark generation.

Geological facts

Chert is extremely durable and weather-resistant, which is why ancient stone tools made from it survive well in the archaeological record. Varieties of chert include flint, jasper, and agate.

Field identification & locations

Can be identified in the field by its hard conchoidal fracture, waxy luster, and lack of effervescence when tested with acid. Commonly found in limestone and chalk formations worldwide.