Chert (Jasper variety)

Physical Properties

Hardness: 6.5-7 Mohs; Color: Tan/Grey with reddish iron staining; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline; Fracture: Conchoidal; Specific Gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich microfossils (radiolaria or diatoms) or chemical precipitation from groundwater within sedimentary rock layers. Frequently found in limestone or dolomite as nodules.

Uses & Applications

Historically used for stone tools and arrowheads; currently used in jewelry (lapidary), as a flint-replacement for fire starting, and as construction aggregate.

Geological Facts

Because it breaks with a conchoidal (shell-like) fracture, it was one of the most important materials for early humans to create sharp cutting tools. It is virtually immune to chemical weathering.

Field Identification & Locations

Look for a smooth, waxy texture on fresh surfaces and edges that are sharp enough to cut skin. It cannot be scratched by a steel knife. Common in river beds and sedimentary outcrops globally.

Identified on: 5/3/2026

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