Chert

Physical Properties

Hardness: 7 (Mohs scale); Color: Gray, green, brown, tan; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formation & Geological History

Formed from the recrystallization of silica-rich microfossils (like radiolarians or diatoms) on the seafloor, or via chemical precipitation from hydrothermal fluids. Commonly found in limestone or dolostone layers.

Uses & Applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp conchoidal fracture. Today used as road base, construction aggregate, and occasionally for lapidary work.

Geological Facts

Chert is so hard it can strike sparks against steel. This specific specimen shows 'knapping' features similar to a lithic flake or debitage, suggesting it may have been historically worked by humans.

Field Identification & Locations

Identify by its waxy texture, hardness (it will scratch glass), and its characteristic shell-like (conchoidal) curved fracture surfaces. Found worldwide in sedimentary basins.

Identified on: 4/21/2026

Mode: Standard