How do geodes form?

Vugs are cavities that might contain crystals, but they do not have a competent lining that allows them to separate from their host rock. Unlike many geodes, they probably will not exist after the host rock is weathered away. Concretions are solid aggregates composed of sediment grains and a cementing material. read more

Geodes can also form in sedimentary rocks such as limestone or sandstone. The cavity in these rocks is usually formed from a solid core. A mass of minerals, or nodule, in the sediment may begin to dissolve and leave space behind. read more

Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks. They can form in gas bubbles in igneous rocks, such as vesicles in basaltic lavas; or, as in the American Midwest, in rounded cavities in sedimentary formations. read more