sedimentary

Chert (Yellow/Brown variety)

Chert (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Yellowish-brown to tan; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Hardness: 6
Color: Yellowish-brown to tan
Luster: Waxy to dull

Identified March 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or by chemical precipitation in marine or groundwater environments. It often replaces limestone or occurs as nodules in sedimentary strata.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp conchoidal edges. Currently used in road construction, for fire-starting (flint), and occasionally as an ornamental stone in jewelry.

Geological facts

Chert is a high-silica rock so tough that it can spark when struck against steel. The yellow or brown coloring is typically caused by the inclusion of iron oxides (limonite/hematite). Many prehistoric cultures traded this specific material across hundreds of miles.

Field identification & locations

Identify in the field by its smooth, waxy texture and shell-like (conchoidal) fracture pattern. It cannot be scratched by a steel knife. Commonly found in limestone outcrops, riverbeds, and glacial till throughout the Midwestern United States and Europe.