The crystals are formed by the filling of vesicles in volcanic and sub-volcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same, or other, minerals precipitated from water, groundwater or hydrothermal fluids. Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks. They can form in gas bubbles in igneous rocks, such as vesicles in basaltic lava; or, as in the American Midwest, in rounded cavities in sedimentary formations.
After rock around the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber.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 common in some formations in the United States (mainly in Indiana, Iowa, Missouri, western Illinois, Kentucky, and Utah). In Oregon they are referred to as thundereggs (which are normally completely filled solid) and are the Oregon State Rock. They also are common in Brazil, Namibia, and Mexico. This item is in the category "Collectibles\Rocks, Fossils & Minerals\Crystals & Mineral Specimens\Display Specimens". The seller is "rocksology" and is located in this country: US. This item can be shipped to United States.