Article
Article
Geode
Article By:
Siever, Raymond Formerly, Department of Geology, Harvard University, Cambridge, Massachusetts.
Last reviewed:April 2021
DOI:https://doi.org/10.1036/1097-8542.286000
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- Geode, published November 2019:Download PDF
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A roughly spheroidal hollow body, lined on the inside with inward-projecting small crystals. Geodes are found most frequently in limestone beds but may occur in some shales. Typically, a geode consists of a thin outer shell of dense chalcedonic silica and an inner shell of quartz crystals, sometimes beautifully terminated, pointing toward the hollow interior (see illustration). Many geodes are filled with water; others, having been exposed for some time at the surface, are dry. Calcite or dolomite crystals line the interior of some geodes, and a host of other minerals are less commonly found. In some geodes there is an alternation of layers of silica and calcite, but almost all geodes show some banding suggestive of rhythmic precipitation. See also: Chalcedony
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