Discovered in 1999 inside an abandoned mine in Southern Spain, Pulpí Geode is the largest crystal 'cave' of its kind in the world. But what exactly is a geode and how do crystals form inside them?

And if you go to Put-in-Bay, you can tour the island's geode for yourself. Read more: 25 Gorgeous Islands For Vacationing That Won't Break The Bank Inside the world's largest geode, which can ...

Geodes are hollow rock formations often with quartz crystals, such as amethyst, inside. Amethyst geodes in Uruguay have been found in cooled lava flows, which date from the original breakup of the ...