Chert (Jasper variety)

Physical Properties

Hardness: 6.5-7 Mohs; Color: Brown, reddish-brown, grey; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or by chemical precipitation from silica-rich fluids in limestone or prehistoric marine environments.

Uses & Applications

Historically used for making stone tools/arrowheads. Currently used as an ornamental stone, in jewelry (cabochons), and for road construction or landscaping aggregate.

Geological Facts

Because it breaks with a conchoidal fracture like glass, it creates extremely sharp edges; it was one of the most important minerals for early human technology. It can also produce sparks when struck against steel.

Field Identification & Locations

Look for a smooth, waxy texture and shell-like (conchoidal) curves where the rock has broken. It is often found in nodules within limestone or as river-worn pebbles. It cannot be scratched by a steel knife.

Identified on: 4/30/2026

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