sedimentary

Geode (unopened nodule)

Siliceous or Calcareous Concretion/Geode (SiO2 or CaCO3)

AI-generated identification · may be incorrect

Hardness: 6.5-7 on Mohs scale (if chalcedony shell) or ~3 (if limestone rind); Color: Tan, brownish-grey, chalky white exterior; Luster: Dull, earthy crust; Shape: Sub-spherical to irregular knobby nodule; Specific Gravity: Relatively light if hollow inside (~2.0-2.65 g/cm3)

Hardness: 6
Color: Tan, brownish-grey, chalky white exterior
Luster: Dull, earthy crust

Identified October 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed as mineral precipitation inside cavities created by gas bubbles in volcanic rocks or dissolved organic matter/fossils in sedimentary carbonate layers (like limestone or shale), followed by slow deposition of microcrystalline silica or calcite over millions of years.

Uses & applications

Popular for lapidary arts, mineral collecting, education, cracking open to reveal crystal interiors (often quartz, calcite, or chalcedony), and decorative displays.

Geological facts

Geodes are famous for looking completely ordinary or like rough, bumpy 'cannonballs' on the outside, while concealing hollow pockets lined with glittering crystals inside.

Field identification & locations

Identified in the field by their rough, rounded cauliflower-like exterior texture, knobby rind, and unusually light weight compared to a solid rock of the same size. Commonly found in regions like Keokuk (Iowa/Missouri/Illinois), Indiana, Mexico, and Morocco.