sedimentary

Chert Nodule (with Geode cavity)

Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 Mohs; Color: Brown, tan, and grey with iron staining; Luster: Waxy to dull; Structure: Microcrystalline; Cleavage: None (Conchoidal fracture).

Hardness: 6
Color: Brown, tan, and grey with iron staining
Luster: Waxy to dull

Identified July 3, 2026More sedimentary →

Explore Chert Nodule (with Geode cavity) 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 grey with iron staining; Luster: Waxy to dull; Structure: Microcrystalline; Cleavage: None (Conchoidal fracture).

Formation & geological history

Formed as rounded masses of silica that precipitated in sedimentary rock layers, such as limestone or chalk, often through the replacement of organic material or filling of voids. Typically Cretaceous to Carboniferous in age.

Uses & applications

Used historically for tool-making due to its sharp edges; today used for lapidary work, decorative garden stones, and as geological teaching specimens.

Geological facts

Chert nodules like this one were the primary source of flint for prehistoric humans. The hollow centers occasionally contain small quartz crystals, forming a mini-geode.

Field identification & locations

Identify by its waxy texture, extreme hardness (it will scratch glass), and rounded, pebble-like exterior. Commonly found in riverbeds and eroded limestone cliffs.