A geode is a geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter are secluded. The crystals are formed by the filling of vesicles in volcanic and sub-volcanic 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.
