World's most valuable stamp to go under the hammer The world's most valuable object by size and weight - the British Guiana One-Cent Black on Magenta stamp - went on display at Sotheby's in London on Monday ahead of it's auction later this month, expected to raise as much as 20 million dollars. Made in 1856 it was discovered by a school boy in 1873 and has since been fiercly guarded as the most valuable stamp ever. Images and soundbites.