http://www.findarticles.com/p/articl...57/ai_66681236
Found a better explanation of gypsum crystal formation for a similar site in Spain.
Geologists think the gigantic geode was formed 6 million years ago, when mineral-saturated water flowed through a rock hole. As water slowly evaporated, minerals clung to the rock's surface, forming a lattice of smooth-faced, geometrical shapes. "When minerals separate from water they stick to solids, just like sugar from a solution sticks to a piece of string," says Paul Burger, a geologist at New Mexico's Carlsbad Caverns. "Crystal formation uses the same process that makes rock candy."
I'm not sure if the subject pictures qualify as a geode, but the mineral deposit and crystallization process would be the same.