sedimentary

Chert

Chert (Microcrystalline Quartz, SiO2)

Mohs hardness: 7. Color: Tan, brown, yellowish-khaki, beige. Luster: Waxy to dull. Crystal structure: Microcrystalline/cryptocrystalline (trigonal quartz). Cleavage: None (conchoidal fracture). Specific gravity: ~2.6.

Hardness: 7
Color: Tan, brown, yellowish-khaki, beige
Luster: Waxy to dull

Identified July 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily through biochemical or chemical precipitation of silica-rich fluids within marine or lacustrine sedimentary rocks, often replacing carbonate rocks or forming nodules in limestone/chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp conchoidal fracture. Modern uses include aggregate in road construction, landscaping, and flint-and-steel spark generation.

Geological facts

Chert is essentially microcrystalline quartz. Darker varieties are called flint, while reddish varieties with iron oxide are called jasper. It was one of the most critical resources for prehistoric humans.

Field identification & locations

Identified by its high hardness (scratches glass), waxy appearance, smooth texture, and characteristic curved, razor-sharp conchoidal fracture surfaces. Commonly found in riverbeds, gravel deposits, and limestone outcrops worldwide.