sedimentary
Geode Nodules / Chalcedony Geode
Silicon Dioxide (SiO2) - Amorphous to Cryptocrystalline Quartz
Hardness: 6.5–7 (Mohs scale); Color: Externally grey, brown, or tan; Luster: Dull/earthy on the exterior; Structure: Spherical or subspherical with a cauliflower-like texture; Specific Gravity: 2.6–2.7.
- Hardness
- 6
- Color
- Externally grey, brown, or tan
- Luster
- Dull/earthy on the exterior
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Physical properties
Hardness: 6.5–7 (Mohs scale); Color: Externally grey, brown, or tan; Luster: Dull/earthy on the exterior; Structure: Spherical or subspherical with a cauliflower-like texture; Specific Gravity: 2.6–2.7.
Formation & geological history
Formed through the hollow filling of cavities in sedimentary or volcanic rock. Over millions of years, mineral-rich groundwater seeps into the cavity, depositing layers of silica that harden into chalcedony or quartz crystals.
Uses & applications
Primarily used for geological education, lapidary arts (cutting and polishing), jewelry (when sliced), and as decorative display pieces for collectors.
Geological facts
Geodes are essentially 'nature’s surprise packages.' Until they are cracked open, one can never be 100% sure if they are solid (nodules) or hollow with internal crystals. The largest geode ever found is the Empress of Uruguay, standing 11 feet tall.
Field identification & locations
Identify in the field by looking for rounded, lumpy, 'cauliflower-textured' rocks that feel unexpectedly light for their size (if hollow). Common in desert environments such as Morocco, Mexico, and the Midwestern USA (Keokuk region).
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