Geodes are geological secondary structures which occur in certain sedimentary and volcanic rocks. Geodes are essentially hollow, spherical to oblate masses of mineral matter that may form from either 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 they are found in) and partial filling by the same or other minerals precipitated from diagenetic or hydrothermal fluids.
Rock Formations, Nature - Misc
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By: kjfitz