 
A geode is a geological formation found inside volcanic and sedimentary rocks. Geodes are hollow, roughly spherical rocks in which masses of mineral crystals are hidden.The crystals are formed in several ways. They can be accretions in volcanic rocks by minerals deposited from hydrothermal fluids (hot groundwater that circulates within the Earth's crust). They can also be created by minerals precipitated from groundwater. 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 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. Over time, this slow accretion 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 at the surface.  When these rocks are cut in half, visible bands corresponding to the stages of accretion may show patterns that reveal the original points of fluid entry into the cavity, as well as various amazingly beautiful colours based on the chemistry of the fluids.Iron oxide (rust) will impart brownish hues to quartz geodes. Most geodes contain clear quartz crystals, but others have purple amethyst crystals. Some can have agate, chalcedony, or jasper, or crystals such as calcite, dolomite, and celestite. There is no easy way of telling what the inside of a geode holds until it is cut open. However, geodes from a particular area are usually similar in appearance. Geodes and geode slices are sometimes dyed with artificial colours. Samples of geodes with unusual colours or highly unlikely formations have usually been synthetically altered. Interested in obtaining geodes to break open yourself? They're available from Amazon here. |