Chert (Jasper variety)

Physical Properties

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

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or inorganic precipitation from silica-rich fluids in sedimentary basins. Primarily Paleozoic to Cenozoic eras.

Uses & Applications

Historically used for stone tools/arrowheads. Currently used in jewelry (as Jasper), lapidary arts, and as an industrial abrasive or construction aggregate.

Geological Facts

Chert is so hard it can create sparks when struck against steel. Prehistoric humans highly valued it for its predictable conchoidal fracturing, which allowed them to knap sharp edges for survival.

Field Identification & Locations

Identify in the field by its hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture pattern. Commonly found in river beds or as nodules in limestone.

Identified on: 4/26/2026

Mode: Standard