sedimentary
Geode or Concretion
Silica-filled nodule (likely SiO2)
Hardness: 6.5-7 (if quartz/agate center); Color: Dull brownish-gray exterior, potential crystalline interior; Luster: Earthy exterior; Crystal system: Trigonal (if quartz); Cleavage: None.
- Hardness
- 6
- Color
- Dull brownish-gray exterior, potential crystalline interior
- Luster
- Earthy exterior
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Physical properties
Hardness: 6.5-7 (if quartz/agate center); Color: Dull brownish-gray exterior, potential crystalline interior; Luster: Earthy exterior; Crystal system: Trigonal (if quartz); Cleavage: None.
Formation & geological history
Formed in sedimentary rock cavities (such as limestone) or volcanic ash beds through groundwater mineral precipitation over millions of years.
Uses & applications
Primarily used as lapidary material, decorative display pieces, or for education/collecting once sliced or cracked open.
Geological facts
Until opened, it is often impossible to know if a geode contains beautiful crystals or is solid (a nodule/thunder egg).
Field identification & locations
Identify by a rounded, bumpy, 'cauliflower' exterior texture that feels lighter than a normal rock of its size. Common in the American Midwest and desert regions.
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