sedimentary

Chert Geode or Nodule

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 (Mohs scale), Color: tan, orange-brown, or rusty red due to iron staining, Luster: dull to waxy, Crystal Structure: trigonal (microcrystalline), Cleavage: none, fracture is conchoidal.

Identified April 9, 2026More sedimentary →

Explore Chert Geode or Nodule 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 (Mohs scale), Color: tan, orange-brown, or rusty red due to iron staining, Luster: dull to waxy, Crystal Structure: trigonal (microcrystalline), Cleavage: none, fracture is conchoidal.

Formation & geological history

Formed primarily through the chemical precipitation of silica-rich fluids within cavities of sedimentary rock layers, such as limestone or shale. These nodules often accumulate over millions of years as replacement minerals.

Uses & applications

Used historically for tool-making (flint knapping) due to its sharp edges when broken. Today, they are popular for rock tumbling, lapidary cross-sections, and as collector specimens when hollow or containing crystals inside.

Geological facts

While the outside looks like a common rock, these nodules are often 'geodes' that contain quartz crystals or chalcedony inside. Chert is so durable that it often survives long after the surrounding host rock has eroded away.

Field identification & locations

Identify by its characteristic rounded, bumpy 'cauliflower' texture and significant weight relative to size. Can be found in gravel pits, stream beds, or weathered out of limestone cliffs. Checking for hollow sounds when tapped can indicate a geode.