Geode

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Geode, halved and polished

Geodes (Greek γεώδης - ge-ōdēs, "earthlike") are geological secondary sedimentary structures which occur in sedimentary and certain volcanic rocks. Geodes are essentially spherical masses of mineral matter that were deposited syngenetically 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. Both structures had the minerals contained within, deposited from groundwater or hydrothermal processes. The exterior of most geodes generally consists of chalcedony, a cryptocrystalline form of quartz. Inside the chalcedony shell 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. Geodes are found mostly in basaltic lavas and limestones. The Warsaw Formation in the Keokuk region near the area where Missouri, Iowa, and Illinois join contains abundant geodes.