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Geodes (Break-Open & Halves)
Geodes are geological rock formations which occur in sedimentary and certain volcanic rocks. Geodes are essentially rock cavities or vugs with internal crystal formations or concentric banding. The exterior of the most common geodes is generally limestone or a related rock, while the interior contains quartz crystals and/or chalcedony deposits. Other geodes are completely filled with crystal, being solid all the way through. These types of geodes are called nodules.
Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks, while the more general term "vug" is applied to cavities in fissures and veins. They can form in gas bubbles in igneous rocks, such as vesicles in basaltic lavas, or as in the American Midwest, 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.
Most geodes contain clear quartz crystals, while others have purple amethyst crystals. Still others can have agate, chalcedony, or jasper banding or crystals such as calcite, dolomite, celestite, etc. There is no easy way of telling what the inside of a geode holds until it is cut open or broken apart. However, geodes from any one locality usually have a more restricted variety of interior mineralization.
Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, Kentucky, and Utah). They also are common in Brazil, Namibia, and Mexico. A large geode was discovered in Put-in-Bay, Ohio in the early 20th century. It is known as Crystal Cave, and tours are possible during the summer. In 1967, Iowa designated the geode as the official state rock, and it has a Geode State Park.
In 2000, a team of geologists found a cave filled with giant gypsum crystals in an abandoned silver mine near Almerķa, Spain. The cavity, which measures 1.8 × 1.7 metres and is 8 metres in length, would be the largest geode ever found. The entrance of the cave has been blocked by five tons of rocks, and is under police protection (to prevent looters from entering). According to geological models, the cave was formed during the Messinian salinity crisis 6 million years ago, when the Mediterranean sea evaporated and left thick layers of salt sediments (evaporites). The cave is currently not accessible to tourists.