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.
Geode (Greek γεώδης - ge-ōdēs, "earthlike") are geological secondary structures which occur in certain sedimentary and volcanic rocks. They are themselves of sedimentary origin formed by chemical precipitation.