How geodes formed from dolomite is a more complicated matter. Geologist Robert Maliva, formerly of Harvard University and now a consulting hydrogeologist, came up with a theory in 1987, which explains most geode features.
It goes like this: The geode cavity formed in a roundabout way — from a hard little nodule that later dissolved away, leaving a void. The process started about 350 million years ago when limey sediments built up in warm, shallow seas that covered what is now central United States. The salty warm waters seeped through calcite-rich limey sediment; the salt reacted with the calcite (which is an ingredient in limestone, chalk and marble), and transformed the calcite into the minerals: dolomite and anhydrite. That's how dolomite enters the picture.
The anhydrite (related to the gypsum used to make wallboard and plaster) is important, too: it formed hard spherical nodules within the dolomite as the dolomite solidified from sediment into rock. Now, the interesting part. Water that is slightly acidic will dissolve anhydrite. As the water percolating through the dolomite became acidic over time, the acidic, mineral-laden water dissolved the outer edges of the anhydrite nodules and replaced them with small fibrous crystals of quartz. The interiors of the anhydrite nodules also dissolved away, "leaving a spherical void with a rind of the tiny quartz crystals," says Smith. The pre-geode then had its cavity.
Mineral-rich water moving through the rock provided quartz (and other minerals in some cases); the crystals inside the cavity grew inward.
Later, the younger rocks eroded away, and exposed the dolomite, which then largely weathered away. The hard quartz resisted weathering, separated from the dolomite and accumulated on the ground as geodes.