Geode

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There is also a line of microprocessors called Geode.

Geode, halved and polished

Geodes (Greek geoides, "earthlike") 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 though. These types of geodes are called nodules.

Geodes can form in any cavity that is buried. These can be gas bubbles in igneous rocks, pockets under tree roots, vesicles in lava after a volcanic eruption, or even animal burrows. Over time, the external wall of the cavity hardens, and 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. Then, over millions of years, the geode makes its way back to the surface through normal geologic processes.

The size of the crystals, including their form and shade of color, vary—making each geode unique. Some are clear as quartz crystals, and others have rich purple amethyst crystals. Still others can have agate, chalcedony, or jasper crystals. There is no way of telling what the inside of a geode holds until it is cut open or broken apart.

Geodes are common in some formations in Indiana, Iowa, Missouri, Kentucky and Utah (in the United States). They also are common in Brazil, Namibia and Mexico. A large geode roughly the size of a minivan 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.

Geodes and geode slices are sometimes dyed with artificial colors. Samples with unusual colors and highly unlikely formations have usually been synthetically altered.

[edit] See also

Lithophysa
Thunderegg

[edit] External links

Categories: Rocks | Mineralogy | Petrology