sedimentary

Chert (Chalcedony/Agate)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 Mohs; Color: Gray, bluish-gray, earthy brown; Luster: Waxy to vitreous; Crystal Structure: Trigonal (cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65.

Hardness: 6
Color: Gray, bluish-gray, earthy brown
Luster: Waxy to vitreous

Identified June 19, 2026More sedimentary →

Explore Chert (Chalcedony/Agate) 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 primarily through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation in marine or groundwater environments. This specimen appears water-worn, likely from a fluvial or glacial deposit.

Uses & applications

Historically used for making stone tools (arrowheads). Modern uses include tumbled stones for jewelry, aquarium decor, and as a component in construction aggregate.

Geological facts

Chert is so hard it will spark when struck against steel. It was one of the first minerals used by early humans due to its predictable, sharp fracturing patterns.

Field identification & locations

Identify in the field by its waxy luster, smoothness when wet, and curved (conchoidal) shell-like chips on the edges. Commonly found in river beds and gravel pits.