Geodes are geological secondary structures which occur in certain sedimentary and volcanic rocks. They are themselves of sedimentary origin formed by chemical precipitation. Geodes are essentially hollow, vaguely spheroid to oblate masses of mineral matter that may have formed via either of two different processes:
by the filling of vesicles (gas bubbles) in volcanic to sub-volcanic rocks by minerals deposited from hydrothermal fluids or
by the dissolution of sedimentary nodules or concretions (that were deposited syngenetically within the rock formations in which they are found) and partial filling by the same or other minerals precipitated from diagenetic water, groundwater or hydrothermal fluids.
When dissected in half, visible bands corresponding to varied stages of precipitation may at times show patterns that reveal point/s of fluid entry into the cavity and/or varied colors corresponding to changes in chemistry.