Certainly not the most valuable object by weight in the known universe

It’s second. Antimatter is first at well over £20 billion per gram. Yet at just 0.02675g, the Swedish Treskilling Yellow is well ahead of its nearest rival, the isotope Californium 252. Last sold (for an undisclosed amount but believed to be well over £3 million) in 2013, this stamp derives its exceptional value from the fact that there is only one known example in existence. It was recently on display at Stampex in Sweden where one of our dealers, Gabriel Cohen, saw it.

A brief printing error sometime in 1857 saw what should have been a blue-green three skilling stamp produced in yellow-orange. It was thought that all the misprinted stamps were destroyed but thirty years later a young collector found one on the cover of a letter in his grandmother’s attic. He sold it to a fellow collector for the equivalent of what would be a couple of pounds today. Its auction history ever since then has set several records.

Although the title for the most expensive stamp ever sold still belongs to the British Guiana 1c Magenta (sold for £7.5m in 2014), its greater weight gives the Swedish contender the edge for now. However, the fact that just one copy of either stamp exists makes this a needless comparison. In that sense, they are both quite priceless – unless you happen to have any bags of antimatter lying around of course.

Although the Treskilling Yellow certainly won’t be on sale at the Market tomorrow, who knows what else you might discover.