sedimentary
Quartz Geode
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Tan/white exterior with possible white or clear interior, Luster: Vitreous (internal) and dull/earthy (external), Crystal Structure: Trigonal, Cleavage: None, Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Tan/white exterior with possible white or clear interior, Luster: Vitreous (internal) and dull/earthy (external), Crystal Structure: Trigonal, Cleavage: None, Specific Gravity: 2.65
Formation & geological history
Formed as hollow cavities in sedimentary or volcanic rock that are filled over time by the precipitation of mineral-rich water. These can be millions of years old, often dating to the Cretaceous or Pennsylvanian periods.
Uses & applications
Used primarily for interior decor, educational specimens, lapidary art, and jewelry if crystals are high quality.
Geological facts
Geodes are like nature’s surprise boxes; while the outside looks like a common lumpy rock, the inside can contain vibrant crystals. The Keokuk geode from Iowa is one of the most famous varieties.
Field identification & locations
Look for rounded, lumpy nodules that feel lighter than they look (indicating a hollow center). Common in river beds and desert environments through the midwestern and southwestern US.
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