sedimentary
Geode (unopened)
Calcedony-lined Geode (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Tan to brownish-gray exterior with a pitted or 'cauliflower' texture; Luster: Dull/earthy exterior; Structure: Spherical to subspherical secondary structure; Cleavage: None (conchoidal fracture when broken)
- Hardness
- 6
- Luster
- Dull/earthy exterior
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Tan to brownish-gray exterior with a pitted or 'cauliflower' texture; Luster: Dull/earthy exterior; Structure: Spherical to subspherical secondary structure; Cleavage: None (conchoidal fracture when broken)
Formation & geological history
Formed as secondary structures in volcanic or sedimentary rocks. Soluble minerals are leached out, creating a cavity where silica-rich fluids precipitate crystals over thousands of years.
Uses & applications
Primarily used as collector specimens, educational items, or cut and polished for lapidary work and home decor.
Geological facts
A geode is essentially a hollow rock with a crystal-lined cavity. You don't know the beauty inside until it's cracked open or sawn; they can contain quartz, amethyst, calcite, or celestite.
Field identification & locations
Identify in the field by their rough, cauliflower-like exterior and unusually light weight for their size (suggesting a hollow center). Common locations include the Midwest USA (Keokuk), Brazil, and Morocco.
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