Geodes are rough rocks filled with crystals, with theories on their formation including bubbles in volcanic rock or cavities created by roots or burrows. They can contain various crystals, with amethyst geodes being particularly beautiful. The world’s largest geode was discovered in Spain in 1999, lined with pure white gypsum crystals and formed around six million years ago. It is currently only accessible to scientists and geologists.
A geode is a rock, rough and unimpressive in appearance, that is partially or completely filled with crystal. The prefix geo means earth, and the rough lumps on the outer surface are often spherical, resembling small planetoids.
Petrologists don’t have a clear understanding of how a geode is formed. The round geodes are theorized to have formed from bubbles in volcanic rock, while the crystals formed later, with the slow infiltration of mineral-laden water into the bubble. The irregularly shaped geodes perhaps form in cavities created by ancient roots or burrows.
Any number of precipitating crystals can form within a geode. The smooth crystal like agate can completely fill the cavity, and the divided agate geodes of various colors make attractive bookends for rockhounds.
Perhaps the most beautiful geode is an amethyst geode, filled with deep purple amethyst crystals all facing inward. Amethyst geodes are mined in Brazil and Uruguay, with Uruguay producing geodes of the deepest purples.
A number of sites claim to be home to the world’s largest geode; Put-in-Bay, Ohio and Jacob’s Cave in Missouri are two locations in the United States that claim the honor. A recent find in Spain dwarfs these sites, where a huge geode was discovered in an abandoned silver mine in 1999. About eight meters (about 26 feet) long, the geode is lined with pure white gypsum crystals. Some of the facets of the crystal extend a meter or more (3 feet) into the hollow interior. The crystals are thought to have formed around six million years ago, when the Mediterranean Sea largely evaporated, leaving behind quantities of saline to form the seeds around which the giant crystals could form. The site, accessible only via a long trek through the abandoned mine, is currently open only to scientists and geologists.
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