sedimentary

Chert (and Chalcedony)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5–7 on Mohs scale; Color: typically white, tan, gray, or brown; Luster: waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: none (conchoidal fracture); Specific gravity: 2.65.

Hardness: 6
Color: typically white, tan, gray, or brown
Luster: waxy to dull

Identified March 25, 2026More sedimentary →

Explore Chert (and Chalcedony) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or the chemical precipitation of silica in groundwater. It can range in age from Precambrian to Cenozoic, often found as nodules in limestone or as bedded deposits.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp conchoidal fracture. Modern uses include glassmaking, abrasives, road construction aggregate, and occasionally as semi-precious gemstones when colorful (Jasper/Agate).

Geological facts

Chert was one of the first materials used by early humans for fire-starting and weaponry. It is so tough and chemically stable that it often remains behind as 'lag' on the landscape long after the surrounding limestone has weathered away.

Field identification & locations

Identify by its waxy texture, extreme hardness (will scratch glass), and 'clamshell-like' conchoidal fractures. Commonly found in limestone outcrops or as river cobbles. Collectors should look for sharp edges and a smooth, almost plastic-like feel on fresh surfaces.