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Description
Geodes differ from vugs in that the former were formed as early, rounded, structures within the surrounding rock and are often removed intact, whereas vugs are irregularly shaped pockets, voids or cavities within a formation, often along a vein or in breccia. Geodes also differ from "nodules" in that a nodule is a mass of mineral matter that has accreted around the nodule nucleus. Both structures had the minerals contained within, deposited from groundwater or hydrothermal processes. Geodes commonly have a chalcedony (cryptocrystalline quartz) shell lined internally by various minerals, often as crystals, particularly calcite, pyrite, kaolinite, sphalerite, millerite, barite, celestite, dolomite, limonite, smithsonite, opal, chalcedony and macrocrystalline quartz, which is by far the most common and abundant mineral found in geodes.

Description
Geodes differ from vugs in that the former were formed as early, rounded, structures within the surrounding rock and are often removed intact, whereas vugs are irregularly shaped pockets, voids or cavities within a formation, often along a vein or in breccia. Geodes also differ from "nodules" in that a nodule is a mass of mineral matter that has accreted around the nodule nucleus. Both structures had the minerals contained within, deposited from groundwater or hydrothermal processes. Geodes commonly have a chalcedony (cryptocrystalline quartz) shell lined internally by various minerals, often as crystals, particularly calcite, pyrite, kaolinite, sphalerite, millerite, barite, celestite, dolomite, limonite, smithsonite, opal, chalcedony and macrocrystalline quartz, which is by far the most common and abundant mineral found in geodes.