The Naica Mine of the Mexican state of Chihuahua is a working mine that is best known for its extraordinary selenite crystals. Located in Naica in the municipality of Saucillo, the Naica Mine, a lead, zinc and silver mine operated by Industrias Peñoles, Mexico's largest lead producer. Caverns discovered during mining operations contain crystals of selenite (gypsum) as large as 4 feet (1.2 m) in diameter and 50 feet (15 m) long.
Read more about Naica Mine: Cave of The Crystals, The Cave of Swords