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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. The minerals contained within a geode have been 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. (source: Wikipedia) Photos by Michael Marcotte Return to List Back to Michael's Hobbies page Back to Michael Marcotte's HomePage