sedimentary
Geode (unopened)
SiO2 - Chert-shelled concretion (mostly Quartz)
Hardness: 7 (outer shell); Color: Dull brownish-gray exterior with possible iron staining; Luster: Dull or earthy exterior; Structure: Spherical or subspherical nodule; Specific gravity: 2.6-2.7.
- Hardness
- 7 (outer shell)
- Color
- Dull brownish-gray exterior with possible iron staining
- Luster
- Dull or earthy exterior
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Physical properties
Hardness: 7 (outer shell); Color: Dull brownish-gray exterior with possible iron staining; Luster: Dull or earthy exterior; Structure: Spherical or subspherical nodule; Specific gravity: 2.6-2.7.
Formation & geological history
Formed in sedimentary rock cavities (often limestone or shale) through the slow deposition of mineral-rich groundwater over millions of years. Common in Mississippian and Pennsylvanian age strata.
Uses & applications
Lapidary use, geological collecting, and education. Once opened, they are often used for home decor or display specimens.
Geological facts
Geodes are virtually indistinguishable from ordinary rocks until cracked open to reveal hollow interiors lined with crystals. The state of Iowa, USA, is world-famous for its 'Keokuk geodes'.
Field identification & locations
Identify in the field by their rough, cauliflower-like exterior and lighter weight compared to solid river stones of similar size. Common in limestone outcrops and stream beds.
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