A cave full of gypsum crystals, some up to a meter-thick, can be found in the Naica mine in Mexico and are spectacular to view. The cave was discovered in 2000, 25 years after the mine chamber had water pumped from it in 1975.
The growth of the crystals has stopped since the mineral-rich waters which provided material for them to grow was removed. The formations, however, have allowed scientists to study them and discover how crystals grow and morph in conditions outside of a laboratory.
You can see more pictures of the gigantic crystals here: http://www.discoverychannel.com.au/images/the-cave-of-crystals/