The 1856 British Guiana one-cent magenta doesn’t look like much. It is a ruddy, squarish, indistinct stamp, its corners chamfered haphazardly. But appearances can be deceiving: Sotheby’s expects to sell it at auction in June for between ten and twenty million dollars. That would make it the most expensive stamp in the world.

As it happens, stamp collecting is in decline; membership in the American Philatelic Society peaked, in 1988, at nearly fifty-eight thousand. By the end of last month, the rolls had shrunk to fewer than thirty-three thousand, even as the U.S. population has increased by nearly a third. The ranks of hardcore collectors, mostly older men, are thinning. For the young, postage stamps can hardly compete with smart phones.

For the most part, stamp prices have also been stagnant or in decline, with some specimens worth less than their value as postage; this is especially true of mass-produced mid-century stamps. “There simply aren’t enough new collectors around to add this type of material to a collection, and most of the established collectors have already filled their album spaces with such stamps,” Charles Snee, the editor of Linn’s Stamp News, told me in an e-mail.

But that’s not true of all stamps. In 1840, Great Britain issued the world’s first adhesive stamp. The prices of specimens issued between then and the early twentieth century are generally on the rise, Snee says. Among them, the one-cent magenta is perhaps the world’s most coveted stamp. As Sotheby’s tells it, in the mid-nineteenth century, the colonial postal system of British Guiana, in South America, relied on stamps imported from England. In 1856, though, a delayed shipment prompted the Guianan postmaster to seek a local supplier. The Royal Gazette newspaper, using a hand press capable of printing just two stamps at a time, was commissioned to produce stamps known as the four-cent blue, the four-cent magenta, and the one-cent magenta. Four cents was the price of a letter; one-cent stamps were used on the wrapping of newspapers, like the Royal Gazette itself. The stamps were so crude that the postmaster initialled each one, to certify that it wasn’t a fake.

People often saved letters and the envelopes in which they arrived, but newspaper wrapping was a different matter—hence the one-cent magenta’s rarity. In 1873, a twelve-year-old Scottish schoolboy named L. Vernon Vaughan, who lived with his family in British Guiana, discovered a one-cent magenta among his uncle’s letters. He sold it for a few shillings, and it eventually made its way into the hands of a rich Austrian count named Philip Ferrari de La Renotière, one of history’s great stamp collectors. The Count bequeathed his collection to Berlin’s Postal Museum, but its repose there was short-lived; after the First World War, according to Sotheby’s, it was seized by France and auctioned in 1922 to help pay Germany’s war reparations. That was how it came to be acquired by the American textile magnate Arthur Hind, for thirty-five thousand dollars (nearly five hundred thousand dollars in today’s money), a record price at the time.

According to a delectable legend that there is no reason to believe, when a second one-cent magenta came into Hind’s possession, he torched it with his cigar to protect the value of the one he already had. Since then, the stamp’s fame has grown: it was featured in a Donald Duck short and a John D. MacDonald mystery. John du Pont, an heir to the DuPont chemical fortune, bought it for nine hundred and thirty-five thousand dollars in a 1980 auction (in today’s dollars, nearly $2.7 million), setting another record. He later died in prison, while serving time for murdering an Olympic wrestler. Sotheby’s is handling the current sale for his estate.

These days, philatelists are no longer likely to be the sole bidders. David Redden, the celebrated Sotheby’s auctioneer and vice-chairman, who will conduct the sale, told me that the prices of trophy properties of all kinds, ranging from a Stradivarius viola to a 1933 double eagle twenty-dollar gold piece, have been soaring, in keeping with the incomes of the supremely wealthy individuals likely to covet them. Sotheby’s is trolling for international one-per-centers by displaying the stamp in Hong Kong and London before bringing it back to New York for auction on June 17th. The dwindling numbers of ordinary stamp collectors may be out of luck.

Photograph: Tyrone Siu/Reuters/Corbis