sedimentary
Geode (Unopened)
Silicon Dioxide (SiO2) in sedimentary host rock
Hardness: 7 (Quartz interior), 5-6 (outer rind). Color: Tan, beige, or grey exterior. Luster: Dull/earthy exterior. Crystal Structure: Trigonal/Amorphous. Specific Gravity: 2.6-2.7.
- Hardness
- 7 (Quartz interior), 5-6 (outer rind)
- Color
- Tan, beige, or grey exterior
- Luster
- Dull/earthy exterior
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Physical properties
Hardness: 7 (Quartz interior), 5-6 (outer rind). Color: Tan, beige, or grey exterior. Luster: Dull/earthy exterior. Crystal Structure: Trigonal/Amorphous. Specific Gravity: 2.6-2.7.
Formation & geological history
Formed in sedimentary rock cavities (such as limestone or shale) through mineral precipitation from hydrothermal fluids or groundwater over millions of years. Common in Mississippian and Devonian strata.
Uses & applications
Primarily used for lapidary work, educational displays, and as a popular item for amateur rock collectors and hobbyists.
Geological facts
The word geode comes from the Greek word 'Geoides' which means earth-like. They look like ordinary rocks on the outside, but can contain spectacular hollow cavities lined with crystals like quartz, amethyst, or calcite.
Field identification & locations
Identify by its unusually light weight (if hollow) and rounded, bumpy, 'cauliflower-like' exterior texture. Commonly found in riverbeds and limestone outcrops in the Midwest USA (Keokuk region) and Brazil.
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