mineral

Geode (Unopened Keokuk Geode)

Silicon Dioxide (SiO2) - Chalcedony / Quartz exterior

Hardness: 6.5 - 7 on Mohs scale. Color: Exterior is tan, beige, brownish, or chalky white, often rough, bumpy, and cauliflower-like. Luster: Dull or earthy exterior, vitreous inside if hollow. Cleavage: None.

Hardness: 6
Luster: Dull or earthy exterior, vitreous inside if hollow

Identified July 27, 2026More mineral →

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Physical properties

Formation & geological history

Geodes form in hollow spaces within sedimentary rocks (like limestone or shale) or volcanic rocks. Minerals slowly precipitate from groundwater over thousands to millions of years, creating a hard outer shell of chalcedony.

Uses & applications

Popular among rockhounds, collectors, and educators. Highly valued for cracking open to reveal hidden internal quartz, amethyst, or agate crystals inside.

Geological facts

The state rock of Iowa is the Keokuk geode. Unopened geodes are often called 'rattlers' if loose crystals inside can be heard shaking. They feel noticeably lighter than solid rocks of similar size if they are hollow inside.

Field identification & locations

Identify in the field by their rough, rounded, spherical or knobby 'cauliflower' texture and lighter weight relative to size. Found extensively in places like Iowa, Missouri, Illinois, Indiana, Mexico, and Brazil.

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