sedimentary

Chert

Microcrystalline quartz (SiO2)

Hardness: 7 on the Mohs scale. Color: various shades of white, grey, brown, yellowish-brown, or black. Luster: waxy to vitreous. Crystal structure: microcrystalline/cryptocrystalline quartz (chalcedony). Cleavage: none (conchoidal fracture). Specific gravity: 2.60 to 2.65.

Hardness: 7 on the Mohs scale
Luster: waxy to vitreous

Identified August 5, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through chemical and biochemical precipitation in marine environments, often replacing limestone or chalk during diagenesis through the accumulation of siliceous skeletal remains of microscopic organisms like radiolarians or diatoms.

Uses & applications

Historically used for making stone tools, arrowheads, and blades due to its sharp conchoidal fracture. Today used in abrasives, road construction, and decorative landscaping.

Geological facts

Chert has been utilized by humans for hundreds of thousands of years for tool-making. Varieties include flint, jasper, and chalcedony. When struck with steel, chert produces sparks, making it vital for fire-starting throughout history.

Field identification & locations

Easily identified by its extreme hardness (scratches glass), waxy luster, and distinctive conchoidal (shell-like) fracture. Commonly found embedded in limestone and chalk formations worldwide.