sedimentary
Geode (unopened) or Calcareous Nodule
Siliceous or Calcareous Concretion/Nodule
Rough, bumpy exterior, dull luster. Often somewhat spherical or knobby. Hardness varies (3 for calcite, 7 for quartz/chalcedony exterior). Color is typically light gray, tan, or chalky white.
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Physical properties
Rough, bumpy exterior, dull luster. Often somewhat spherical or knobby. Hardness varies (3 for calcite, 7 for quartz/chalcedony exterior). Color is typically light gray, tan, or chalky white.
Formation & geological history
Formed in sedimentary rocks like limestone or shale. Minerals precipitate from groundwater into cavities or around a nucleus over millions of years.
Uses & applications
Often collected to be broken or sawed open to reveal potential crystal formations inside. Also sold as novelty items.
Geological facts
Geodes look like ordinary, ugly rocks on the outside but can contain beautiful crystals (like amethyst or clear quartz) on the inside.
Field identification & locations
Identify by its spherical or knobby shape, rough exterior somewhat cauliflower-like, and feeling relatively lighter than a solid rock if hollow. Found in sedimentary deposits worldwide, notably in the midwestern US.
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