7x5 Inch (18x13cm) Print - High quality RA4 prints. ^BCave of Crystals.^b Geologist in the Cave of Crystals (^ICueva de los Cristales^i) in Naica Mine, Chihuahua, Mexico. The crystals are the largest known in the world, and are formed of the selenite form of gypsum (calcium sulphate). They formed over millions of years in the mineral-rich geothermally heated water that filled the caves. The crystals were discovered after the water was pumped out of the mine. The Cave of Crystals is 290...