A rare one-cent magenta postage stamp, printed in British Guiana in 1856 and most recently owned by the estate of murderer John du Pont, will be the most valuable object by weight and size ever sold.

It will go under the hammer in New York this June. It's expected to fetch nearly $20 million at the Sotheby's auction.

On March 17, 2014, the expert committee at the Royal Philatelic Society London - the oldest and most distinguished society of stamp scholars in the world - convened a special meeting with the singular purpose of re-authenticating the unique British Guiana One-Cent Magenta.

After close examination by each of the panel's six noted experts, including spectrometer analysis, it has once again certified the British Guiana as genuine making it "the world's most famous stamp."

The last time they examined and certified the world's most famous stamp was in 1935, marking a new generation of experts that have confirmed its place at the apex of the philatelic world. It said that no stamp is rarer than the British Guiana - a unique yet unassuming penny from 1856, and no stamp is more valuable: each of the three times it has been sold at auction, it has established a new record price for a single stamp.