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 More sedimentary →
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Physical properties
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).
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.
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