sedimentary
Geode (Unopened)
Silica-rich concretion (SiO2)
Hardness: 7 (Mohs scale) if quartz-filled; Color: Gray, brown, or tan exterior with a lumpy, cauliflower-like texture; Luster: Dull/earthy exterior; Crystal structure: Usually hexagonal (quartz) or chalcedony inside; Cleavage: None/Conchoidal.
- Hardness
- 7 (Mohs scale) if quartz-filled
- Luster
- Dull/earthy exterior
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Physical properties
Hardness: 7 (Mohs scale) if quartz-filled; Color: Gray, brown, or tan exterior with a lumpy, cauliflower-like texture; Luster: Dull/earthy exterior; Crystal structure: Usually hexagonal (quartz) or chalcedony inside; Cleavage: None/Conchoidal.
Formation & geological history
Formed through the precipitation of minerals into hollow cavities inside sedimentary or volcanic rocks. Usually formed by gas bubbles in cooling lava or dissolved organic matter in sedimentary layers over millions of years.
Uses & applications
Commonly used for geological education, lapidary art, home decor, and as a popular item for amateur rock collectors to break open.
Geological facts
Geodes are essentially 'nature's surprise packages'; while the outside looks like a common rock, the inside can contain amethyst, citrine, calcite, or celestite. Some geodes are over 250 million years old.
Field identification & locations
Identify in the field by looking for round or egg-shaped rocks with a bumpy 'cauliflower' surface that feel lighter than they look (if hollow). Common in the Midwestern USA (Iowa, Illinois) and Brazil.
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