sedimentary

Geode Nodules / Agate Chert

Silicon Dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 (Mohs). Color: Dusty grey-brown exterior with iron staining. Luster: Dull waxy exterior. Crystal structure: Cryptocrystalline. Specific gravity: 2.6-2.65.

Hardness: 6
Color: Dusty grey-brown exterior with iron staining
Luster: Dull waxy exterior

Identified June 14, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs). Color: Dusty grey-brown exterior with iron staining. Luster: Dull waxy exterior. Crystal structure: Cryptocrystalline. Specific gravity: 2.6-2.65.

Formation & geological history

Formed through the deposition of silica-rich groundwater within cavities of sedimentary or volcanic host rock. Often found in limestone beds where silica replaces calcite or fills voids.

Uses & applications

Used for lapidary work (when cut and polished), geological study, and as decorative pieces for collectors.

Geological facts

Geodes are often called 'thunder eggs' when they are solid or filled with agate. This specimen's cauliflower-like exterior is a classic sign of a silica nodule that may contain crystals inside.

Field identification & locations

Look for rounded, bumpy, or 'cauliflower' textured stones that are unusually heavy or light for their size. Common in the Midwest USA (Keokuk region) and Brazil.