“The largest natural crystals on Earth have been discovered in two caves within a silver and zinc mine near Naica, in Chihuahua, Mexico,” a Discovery News article reprinted here tells us. “Reaching lengths of over 20 feet, the clear, faceted crystals are composed of selenite, a crystalline form of the mineral gypsum.”
If you're thinking of exploring the Naica caves, you might want to bring along your own climate controlled biosuit and an ample supply of oxygen as the temperatures inside are an unbearable 150˚F with 100% humidity. It will be like walking into a blast furnace. One death has already been recorded, that of a miner trying to steal some of the crystals: “He tried to take some plastic bags filled with fresh air inside, but the strategy didn't work. He lost consciousness and later was found thoroughly baked.”
There are now plans to air-condition the caves.