A geode (/ˈdʒiː.oʊd/; from Ancient Greek γεώδης (geṓdēs) ´earthlike´) is a geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which ...
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masses of mineral matter (which may include crystals) are secluded. The crystals are formed by the filling of vesicles in volcanic and subvolcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same or other minerals precipitated from water, groundwater, or hydrothermal fluids.