The world's largest rhodochrosite crystal -- so valuable it's been nicknamed the Mona Lisa of crystals -- has been uncovered in a mine in Colorado. A slew of museums, including the Smithsonian, want it, but the man who found the rock has made a deal for it to go on display at the Denver Museum of Natural History. The crystal is a hunk of red manganese carbonate the size of a brick surrounded by calcite and fluoride crystals in blue and yellow. It's still in the quartz formation in which it grew some 30 million years ago, when the Rocky Mountains first formed. Bryan Lees found it in the Sweet Home Mine, where Lees said more such specimens may still exist.