sedimentary

Geode (Unbroken)

Silicon Dioxide (SiO2) usually as Microcrystalline Quartz or Chalcedony

Hardness: 6.5–7 on the Mohs scale; Color: Tan to grey cauliflower-like exterior; Luster: Dull/earthy exterior, vitreous (glassy) interior; Structure: Spherical or subspherical with a hollow or semi-hollow center.

Hardness: 6
Color: Tan to grey cauliflower-like exterior
Luster: Dull/earthy exterior, vitreous (glassy) interior

Identified March 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in sedimentary rocks like limestone or dolomite (and sometimes volcanic ash) when mineral-rich groundwater precipitates crystals within pre-existing cavities. Common in strata from the Mississippian and Pennsylvanian periods.

Uses & applications

Primarily used for geological education, lapidary arts, mineral collecting, and interior decoration once cut and polished.

Geological facts

Geodes are often called 'thunder eggs' when they are solid or volcanic. The word is derived from the Greek 'geoides,' meaning 'earth-like,' referring to their round shape.

Field identification & locations

Identify in the field by their distinct cauliflower-like exterior texture and lighter-than-expected weight if hollow. Common locations include Keokuk (Iowa/Missouri/Illinois), Morocco, and Chihuahua (Mexico). Collectors look for 'rattlers' (loose crystals inside).

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