sedimentary
Geode (Chert or Quartz Nodule)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, beige, or white exterior; Luster: Dull/earthy exterior, vitreous (glassy) interior; Crystal structure: Hexagonal/Trigonal (cryptocrystalline); Cleavage: None (conchoidal fracture).
- Hardness
- 7 (Mohs scale)
- Color
- Tan, beige, or white exterior
- Luster
- Dull/earthy exterior, vitreous (glassy) interior
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, beige, or white exterior; Luster: Dull/earthy exterior, vitreous (glassy) interior; Crystal structure: Hexagonal/Trigonal (cryptocrystalline); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed as secondary fillings in cavities within sedimentary rocks like limestone or dolomite. Silica-rich groundwater percolates through the rock, depositing layers of minerals over millions of years.
Uses & applications
Commonly used for lapidary work, home decor, educational specimens, and crystal healing collections.
Geological facts
Geodes are often nicknamed 'thunder eggs' when they are solid or filled with agate. They are essentially time capsules that contain minerals crystallized thousands to millions of years ago.
Field identification & locations
Identify by its rounded, cauliflower-like exterior texture and surprising lightness (indicating a hollow center). Commonly found in the Midwest USA (Keokuk region) and desert environments.
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