Gypsum, hydrated calcium sulfate, is a common and abundant mineral throughout the world. It occurs near ore deposits, in massive beds, in caves, and in veins where hot mineral solutions have cooled or evaporated. Gypsum cave formations sometimes look like floral arrays, but immense crystals are known from a number of localities. Perhaps the most unique is at Naica, Mexico, where transparent gypsum crystals (called selenite) are found, along with spectacular gypsum crystals that can reach a length of nearly 100 feet. The 'Cave of Swords' at Naica is famous for sprays of excellent gypsum crystals that are 5-10 feet long.