Chert

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: tan, grey, pinkish-brown; Luster: waxy to dull; Crystal Structure: Cryptocrystalline; Cleavage: none (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed primarily through the accumulation of silica-rich shells of microscopic organisms (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids in marine environments. Common in limestone and dolostone formations.

Uses & Applications

Historically used for making sharp tools like arrowheads and scrapers. Currently used in road construction as sub-base material and sometimes in lapidary work for tumbled stones.

Geological Facts

Chert is so durable that it can remain intact long after the surrounding rock has eroded away. It produces sparks when struck against steel, which made it vital for prehistoric fire-making.

Field Identification & Locations

Identify in the field by its hardness (it scratches glass), its waxy feel, and its characteristic curved, shell-like (conchoidal) fracture pattern. Often found as nodules within limestone outcrops.

Identified on: 5/3/2026

Mode: Standard