sedimentary
Geode (unopened)
Silica-filled Concretion (SiO2)
Hardness: 7 (outer shell/internal quartz). Color: Tan, brown, or grayish exterior with pitted 'cauliflower' texture. Luster: Dull/earthy exterior. Structure: Spherical or subspherical nodules.
- Hardness
- 7 (outer shell/internal quartz)
- Luster
- Dull/earthy exterior
Identified More sedimentary →
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Physical properties
Hardness: 7 (outer shell/internal quartz). Color: Tan, brown, or grayish exterior with pitted 'cauliflower' texture. Luster: Dull/earthy exterior. Structure: Spherical or subspherical nodules.
Formation & geological history
Formed in sedimentary rock cavities (like limestone) or volcanic ash. Over millions of years, mineral-rich groundwater seeps into hollow spaces, depositing layers of silica or calcite. These specific 'mudball' types are common in Paleozoic sedimentary layers.
Uses & applications
Primarily for geological collecting, education, and interior decoration once cut and polished. High-quality specimens with amethyst or agate centers are used in jewelry.
Geological facts
The word geode comes from the Greek word 'Geoides' meaning earth-like. Many geodes are millions of years old and remain hollow inside until someone cracks them open.
Field identification & locations
Identify by its rounded, uneven 'cauliflower' exterior and surprising lightness compared to solid rocks. Found globally, but famous in the US Midwest (Keokuk) and Brazil.
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