Chert (Jasper/Agate variety)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown, tan, and gray; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from hydrothermal fluids within sedimentary host rocks. Common in deposits from the Paleozoic to Mesozoic eras.

Uses & Applications

Historically used for stone tools (arrowheads/scrapers) due to its sharp conchoidal fracture. Currently used as an ornamental stone, lapidary material for cabochons, and sometimes as an abrasive or in construction aggregate.

Geological Facts

Chert is so durable that it often survives as rounded pebbles in stream beds long after the surrounding rock has weathered away. Early humans used it for fire-starting by striking it against steel or iron pyrites.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in river beds, gravel pits, and as nodules within limestone.

Identified on: 4/30/2026

Mode: Standard