sedimentary
Geode (Chert/Quartz)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs); Color: Tan/brown exterior; Luster: Dull/earthy exterior; Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture)
- Hardness
- 6
- Color
- Tan/brown exterior
- Luster
- Dull/earthy exterior
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Tan/brown exterior; Luster: Dull/earthy exterior; Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture)
Formation & geological history
Formed in sedimentary rock cavities (like limestone) through chemical precipitation of silica from groundwater. Most are millions of years old.
Uses & applications
Primarily for hobbyist collecting, interior decoration, and educational purposes. Some are used in lapidary work.
Geological facts
The word geode comes from the Greek word Georgos, meaning 'earth-like'. While the outside looks like a common rock, the inside can contain vibrant crystals like amethyst or agate.
Field identification & locations
Identify by its rounded shape and bumpy, cauliflower-like exterior. They feel lighter than solid rocks of the same size if they are hollow. Common in desert regions or limestone beds.
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