sedimentary

Chert Nodules or Concretion

Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Brown, tan, and cream; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); High specific gravity relative to porous rocks.

Hardness: 7 on Mohs scale
Color: Brown, tan, and cream
Luster: Dull to waxy

Identified July 17, 2026More sedimentary →

Explore Chert Nodules or Concretion in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 7 on Mohs scale; Color: Brown, tan, and cream; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); High specific gravity relative to porous rocks.

Formation & geological history

Formed via chemical precipitation of silica-rich fluids within sedimentary rock layers, often replacing organic material or filling voids in limestone or chalk during diagenesis.

Uses & applications

Used historically for stone tools (arrowheads/scrapers) due to its sharp edges when broken; currently used in decorative landscaping and as a collector's curiosity.

Geological facts

Chert is so durable that it often remains behind as 'lag' after the surrounding softer limestone has eroded away. It is often confused with fossilized bone due to its organic shapes.

Field identification & locations

Identify by its hardness (it will scratch glass) and its smooth, waxy texture compared to the surrounding rock. Common in limestone outcrops and riverbeds.