Mystical and alluring, geodes are a curiosity of the natural world. An enclosed and rugged outer shell with a layer of inner crystals, geodes are like a surprise-filled dessert: people are drawn to discovering what crystals are inside a given geode.
Spherical and seemingly magical, a geode begins with a bubble inside a rock and evolves over millennia. Most commonly, the cavity in igneous rock is created when lava or magma solidifies. When cooling, a balloon of carbon dioxide and water vapor can surface and after the gas dissipates, a cavern is left behind in the rock. In other cases, a mass of sediment or organic matter can be stuck in the center and dissolve with time, or a lava pillow forms as lava solidifies underwater.
The geode crystals appear when mineral-plush groundwater or rainwater drips into the rock’s cavity via microscopic pores. From amethyst to quartz filled geodes, the varieties of crystals that formulate depend on the water’s minerals, and conditions including temperature and acidity. And, of course, the passing of time.
In Mexico, near the town of Delicias, Cueva de los Cristales (Cave of Crystals) holds one of the largest known crystal growths in the world: gypsum that stretches as long as 36 feet. Discovered in a portion of the Naica Mine—which extracts lead, zinc, and silver—the Cave of Crystals goes 950 feet underground.
At Heineman Winery in Put-In-Bay, Ohio, one of the world’s largest geodes exists: the Crystal Cave, which was discovered in 1897 when workers were digging a well for the wintery. The cave is covered in a blue-tinted mineral called celestite, and the geode crystals vary from 8 to 18 inches in length. Actually, establishing tours of the cave helped the winery to subsist through prohibition. Today, crystal tours are still open to the public.
In an old silver mine in Pulpi, Almeria, Spain, a crystal-filled chamber was discovered in 2000. The cavern—eight meters in length and 1.8 meters wide—is covered with extremely transparent gypsum crystals, most of which measure a half-meter and some are even two meters.