Half Sphere Geode Specimen. Grey rock surrounds the crystalline formation inside, and a small deposit of black crystal on one side. 

Size: 3 1/2  x 3 3/4 x 2 in. 

A geode is a geological formation within sedimentary and volcanic rocks. They are hollow, vaguely spherical rocks, in which masses of mineral matter which may include crystals are secluded. The crystals are formed by the filling of vesicles in volcanic and subvolcanic rocks by minerals deposited from hydrothermal fluids. After rock around the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface. Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz. When cut in half, visible bands corresponding to varied stages of precipitation may at times show patterns that reveal points of fluid entry into the cavity and/or varied colors corresponding to changes in chemistry. Geodes are found where the geology is suitable with many of the commercially available ones coming from Brazil, Uruguay, Namibia, and Mexico. Geodes are also common in some formations in the United States (mainly in the Midwest and some parts of the Southwest) as well as parts of England and northern Asia. While the term “geode” generally describes hollow formations, if the rock is completely solid inside (often filled with chalcedony) it would be classified as a “nodule” or “thunderegg.”