sedimentary
Geode (Unopened)
Silica Concretion / Geode (Formula: SiO2 for quartz internal contents)
Hardness: 6.5-7 (outer shell); Color: Dull brownish-gray exterior with a cauliflower-like texture; Luster: Earthy (exterior); Crystal structure: Microcrystalline exterior; Specific gravity: 2.6-2.7 (often lighter if hollow).
- Hardness
- 6
- Color
- Dull brownish-gray exterior with a cauliflower-like texture
- Luster
- Earthy (exterior)
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Physical properties
Hardness: 6.5-7 (outer shell); Color: Dull brownish-gray exterior with a cauliflower-like texture; Luster: Earthy (exterior); Crystal structure: Microcrystalline exterior; Specific gravity: 2.6-2.7 (often lighter if hollow).
Formation & geological history
Formed in sedimentary rocks (commonly limestone or shale) through the filling of vesicles or animal burrows by mineral-rich hydrothermal fluids. Usually dating from the Mississippian or Devonian periods (approx. 300-400 million years ago).
Uses & applications
Predominantly used for mineral collecting, lapidary arts, and as decorative home accents/specimens. Sometimes used in jewelry after being cut and polished.
Geological facts
Geodes are often hollow, but if they are filled with solid mineral matter, they are technically called nodules. The state of Iowa, USA, is world-famous for the Keokuk geode beds.
Field identification & locations
Identify in the field by looking for unusually round, bumpy, 'cauliflower-textured' rocks that feel lighter than they look. Found commonlly in riverbeds or limestone outcroppings in regions like the Midwest USA, Mexico, and Brazil.
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