sedimentary

Chert

Microcrystalline silica (SiO2)

Hardness: 7 (Mohs scale); Color: gray, tan, brown; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: gray, tan, brown
Luster: waxy to dull

Identified July 20, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: gray, tan, brown; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed from the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or through chemical precipitation in sedimentary environments. Common in limestone and dolostone beds.

Uses & applications

Historically used for stone tools (flintknapping); currently used in road construction, as an abrasive, and sometimes for landscape decoration.

Geological facts

Chert has a conchoidal fracture, allowing it to break into sharp edges, which made it the primary material for prehistoric arrowheads and spear tips worldwide.

Field identification & locations

Identify in the field by its hardness (scratches glass), waxy appearance on fresh surfaces, and distinctive shell-like (conchoidal) breakage patterns. Found in sedimentary outcrops globally.