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?

Read more: 25 Gorgeous Islands For Vacationing That Won't Break The Bank Inside the world's largest geode, which can contain at least six people at a time, you'll see crystals of 8 to 18 inches ...

Large crystals found in geodes and other rocks are relatively rare. Pink ChalcedonyA pink chalcedony shows off its beauty. Chalcedonies include many types of cryptocrystalline quartz gems and ...