what are geodes

Geodes are hollow, spherical rocks that contain mineral matter inside. They can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks. Geodes can form in gas bubbles in igneous rocks, such as vesicles in basaltic lava, or in rounded cavities in sedimentary formations. After rock around the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface, forming the mineral matter. Geodes can contain a variety of minerals, including clear quartz crystals, purple amethyst crystals, agate, chalcedony, jasper, calcite, dolomite, celestite, and more. The banding and coloration of geodes is the result of variable impurities. Iron oxides will impart rust hues to siliceous solutions, such as the commonly observed iron-stained quartz. Geodes are found where the geology is suitable, with many of the commercially available ones coming from Brazil, Uruguay, Namibia, and Mexico. Large, amethyst-lined geodes are a feature of the basalts of the Paraná and Etendeka traps found in Brazil, Uruguay, and Namibia. Geodes are common in some formations in the United States, mainly in Indiana, Iowa, Missouri, western Illinois, Kentucky, and the desert and central regions.