Geode

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A geode (; from Ancient Greek γεώδης (geṓdēs) 'earthlike') is a geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which 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 syngenetic concretions and partial filling by the same or other minerals precipitated from water, groundwater, or hydrothermal fluids.

Read on Wikipedia · Image Manfred Heyde, CC BY-SA 3.0