sedimentary

Chert / Chalcedony (Agatized)

Silicon dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 Mohs; Color: Brown, tan, and gray swirled; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture).

Hardness: 6
Color: Brown, tan, and gray swirled
Luster: Waxy to dull

Identified June 30, 2026More sedimentary →

Explore Chert / Chalcedony (Agatized) in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 6.5-7 Mohs; Color: Brown, tan, and gray swirled; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture).

Formation & geological history

Formed through the precipitation of silica from groundwater or oceanic fluids, often infilling cavities in limestone or replacing organic material. Typically dated back to various geological eras from Paleozoic to Mesozoic.

Uses & applications

Historically used for stone tools (flint); currently used for lapidary work, tumbling, and as decorative gravel.

Geological facts

This specimen appears to be a water-worn nodule. Chert is extremely durable and chemical resistant, which is why it commonly survives as river gravel long after other rocks erode.

Field identification & locations

Identify by its waxy texture, conchoidal (shell-like) fracture patterns, and hardness (it will scratch glass). Commonly found in riverbeds, glacial till, and sedimentary outcrops.