Geodes are geological secondary sedimentary structures which occur in sedimentary and certain volcanic rocks. Geodes are essentially spherical masses of mineral matter that were deposited sygenetically within the rock formations they are found in. Geodes have a Chalcedony shell containing various minerals, usually quartz. Geodes differ from vugs in that they were formed as a separate entity from the surrounding rock, whereas vugs are voids or cavities within a rock formation. Geodes also differ from nodules in that a nodule is a mass of mineral matter that has accreted around the nodule nucleous. Inside them many minerals have been found such as Calcite, Pyrite, Kaolinite, Sphalerite, Millerite, Barite, Dolomite, Limonite, Smithsonite, and Quartz, which is by far the most common and abundant mineral found in geodes.