sedimentary

Chert

Microcrystalline quartz (SiO2)

Hardness: 7 on the Mohs scale. Color: Varies widely including white, grey, brown, black, and reddish-brown. Luster: Vitreous to dull. Crystal structure: Microcrystalline/cryptocrystalline quartz (chalcedony). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.60 to 2.65.

Hardness: 7 on the Mohs scale
Luster: Vitreous to dull

Identified August 7, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms by the chemical precipitation of silica from aqueous solutions, often derived from the accumulation of siliceous skeletal remains of marine organisms like diatoms and radiolarians, or through the replacement of limestone by silica-rich groundwater.

Uses & applications

Historically used by early humans for making stone tools and weapons (arrowheads, blades, scrapers) due to its conchoidal fracture creating sharp edges; also used for striking fire (flint) and occasionally as decorative aggregate or in abrasives.

Geological facts

Chert that is dark-colored or black is commonly referred to as flint. Because of its extreme durability and weather-resistant properties, ancient stone tools made of chert often survive intact for hundreds of thousands of years.

Field identification & locations

Can be identified in the field by its hardness (scratches glass), waxy to dull luster, and characteristic conchoidal fracture. Commonly found embedded in limestone or chalk formations worldwide.