[catweasel]

If you can't make it to that one you could try Lechuguilla in New Mexico. Also a crystal cave, formed from acid running through limestone, it was featured in David Attenborough's "Planet Earth" series. The prettiest part is the "chandelier ballroom", massive crystals hanging from the ceiling. Access is highly restricted though, you'd probably have to join a queue and go with an official research team. http://en.wikipedia.org/wiki/Lechuguilla_Cave