The reverse side of the British Guiana One-Cent Magenta, described as the most famous stamp in the world, on display in a glass case at Sotheby's June 17, 2014 in New York. No stamp is rarer than the sole-surviving example of the British Guiana One-Cent Magenta, a unique penny issue from 1856, which will be auctioned June 17 in New York with a pre-sale estimate of USD $ 10 million to 20 million. AFP PHOTO/Stan HONDA