Although many desirable stamps are in existence, a handful of rare issues represent the "Holy Grail" for stamp collectors. These stamps have gained legendary status for their notoriety and rarity. Even though they're much too expensive for the average collector to own, each of these rare stamps offers an intriguing story about its origin.
The Inverted Jenny
One of the most famous and recognisable stamps of all time, this 1918 U.S. issue is an obvious error. The 24-cent Jenny stamp was created for the new medium of airmail, so the printing was hurried. During the printing process, one of the sheets went into the press upside-down, resulting in a single sheet of 100 stamps with an inverted plane.
The Red Mercury stamp originally appeared in 1856. It was a newspaper stamp, serving as postage for newspapers in Austria. The Red Mercury, like other newspaper stamps, doesn't show a numerical designation. In 1858, another stamp replaced the Red Mercury. Its relatively short circulation makes it the rarest Austrian newspaper stamp.
The Inverted Swan
One of the earliest examples of a frame invert error, the Inverted Swan is a 4-cent stamp released in Australia in 1855. In January of that year, a damaged printing press and an improper repair led to the production of 388 Inverted Swan stamps. Only 15 complete examples of this stamp with an upside-down swan are known to exist today.
The Benjamin Franklin Z Grill
With only two known copies in existence today, the Benjamin Franklin Z Grill is the rarest and most valuable U.S. stamp. Issued in 1868, it features ink-absorbing squares on the back. This unique design was meant to keep people from washing out the cancellation marks and reusing the stamp.
Post Office Mauritius
Also known as the Blue Penny and the Red Penny, this rare stamp comes in two varieties: a 1-penny and a 2-penny. Both stamps feature the words, "Post Office," instead of the intended "Post Paid." When they were issued in 1847, 240 of these stamps sold before anyone caught the error. Only 26 copies are known to survive today.