What is the rarest stamp?

Answer 1

If the question is about a single stamp, then Sweden's 1855 Three Skilling Banco printed in Yellow (Error of Color) is the rarest and most expensive stamp ever sold to date. Normally printed in green, only one instance of a yellow variety of this stamp was ever found. It turned up in 1885 some 30 years after it was issued. It was printed with the ink meant for the 8 skilling banco of the same set. It seems likely that a 3 skilling die was used to replace a broken 8 skilling die and when the error was caught by the pressman, one or more of the sheets failed to be destroyed. But even if more than 1 copy were ever released, no second copy has ever been found. From this period some 95 percent of all stamps issued were never saved, therefore it is conceivable that there may have been more copies released which were used and then trashed by the recipiant. In 1996 the one and only known copy of the stamp sold at auction for $2.27 million USD.

However, the most expensive single stamp collector's item ever sold is actually a cover with two different rare stamps tied together onto the one cover. In 1847 the British colony of Mauritius printed only 500 examples each of its first two stamps with respective face values of 1d and 2d. In total fewer than 30 of these very rare stamps survive today. In 1993 one cover with both the 1d and the 2d stamps attached side by side and tied together with a single cancelation mark sold for $3.8 million USD, the highest price ever paid for a stamp collecting item.