Chert (Jasper/Flint variety)

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Brown, tan, reddish-brown; Luster: Dull to waxy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the replacement of limestone/dolomite by silica-rich fluids. Can be of any geological age from Precambrian to Cenozoic.

Uses & Applications

Historically used for stone tools and weapons due to sharp edges; currently used in jewelry (lapidary), as road base, and for fire-starting in survivalist gear.

Geological Facts

Chert was one of the most important minerals for early humans because its conchoidal fracture produces razor-sharp edges. When struck against steel, it can create sparks to start a fire.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) breakage patterns. Often found as nodules within limestone or as bedded deposits.

Identified on: 4/20/2026

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