sedimentary

Geode (Unopened)

Siliceous Septarian Concretion / Geode (SiO2)

Hardness: 7 (Mohs scale) for exterior quartz/chalcedony; Color: Brown, tan, and gray exterior with iron staining; Luster: Dull/earthy exterior, likely vitreous interior; Structure: Cryptocrystalline shell, potentially containing macrocrystalline quartz or calcite centers; Cleavage: None (conchoidal fracture).

Hardness: 7 (Mohs scale) for exterior quartz/chalcedony
Color: Brown, tan, and gray exterior with iron staining
Luster: Dull/earthy exterior, likely vitreous interior

Identified March 30, 2026More sedimentary →

Explore Geode (Unopened) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through the precipitation of minerals into a hollow cavity, usually within sedimentary limestone or volcanic ash. This specimen appears to be of the sedimentary variety found in the Midwestern United States, likely dating to the Mississippian subperiod (approx. 340 million years ago).

Uses & applications

Primarily used for geological education, lapidary arts (cutting and polishing), and mineral collecting. High-quality opened geodes are used as decorative home decor or semi-precious jewelry components.

Geological facts

Geodes are often called 'thunder eggs' when associated with volcanic rocks. The Keokuk geodes from the tri-state area of Iowa, Missouri, and Illinois are world-famous for their diversity of interior minerals including quartz, calcite, and pyrite.

Field identification & locations

Identify in the field by looking for rounded, cauliflower-like exterior textures that feel unusually light for their size (indicating a hollow center). Commonly found in stream beds or within carbonate rock outcrops. If it rattles when shaken, it may contain loose crystals inside.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary