How do geodes form?
by 
Erin Albrecht
ANSWERS: 2
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"Geodes are formed in a cavity such as might be found inside a fossil shell buried in sediment. At the beginning, this cavity is probably filled with a concentrated salt solution. The first step in the creation of a geode is the formation along the inner cavity wall of a layer of gelatinous silica, which will eventually be transformed into the chalcedony layer. As the water surrounding the layer becomes less salty, osmosis induces migration of fluids into the cavity. This results in a buildup of pressure, causing the cavity to expand until the water inside and outside is equally salty. When the silica gel dehydrates, crystallizes to form chalcedony, and cracks, mineral-bearing water enters to slowly deposit the inward-projecting crystals". http://www.answers.com/topic/geode
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The other example seems too specific - certainly not all geodes are formed in preexisting cavities. Many seem to require replacement of spherical features such as concretions. The concentric layers and generally increasing crystal size iinward require continual addition of dissolved silica and decelerating rate of crystallization. Not all geodes have evidence of fractures extending to the center, thus exactly how silica enters to fill a geode (molecular diffusion along grain boundaries?) is one of their unanswered questions. See http://www.cst.cmich.edu/users/dietr1rv/thunderegg.htm
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