10. Penny Black
Issued in the UK in May 1840, the Penny Black was the first adhesive stamp in the world, which is why this stamp is considered a valuable stamp. While not the rarest or most valuable collector's item, it charted the future of British postage stamps. The Penny Black bears an image of Queen Victoria II and does not show the country of origin, as is still customary today. While there are a large amount of Penny Blacks for sale, one of these unused stamps can be worth as little as R$12,000, making it a very popular stamp with collectors.
9. Inverted Swan
The Inverted Swan is one of the most famous and exclusive stamps in the world, not because of its beauty, but because it has one of the first inverted errors in the world. The Inverted Swan was published in Perth, Australia, in 1855, when a complicated process of producing these stamps by lithography was used. However, contrary to popular belief, it is actually the frame that is inverted rather than the swan. This famous stamp was last sold in 1983 for R$142,000.
8. Red Mercury
Red Mercury is extremely valuable due to its rarity. Instead of a postage stamp, the Red Mercury was used to mail newspapers. These stamps bear an image of the Roman god and were printed in yellow, red and blue, depending on the number of newspapers in the stack. However, the Red Mercury was short-lived and was quickly replaced, for this reason few copies survive and therefore they have a value of up to R$ 148,000.
7. Missionary Issue Stamps
Despite the cheap blue paper on which the missionary issue stamp was printed, these are the rarest and most valuable stamps of all time, reaching a value of R$156,000. The Missionary Issue Stamps were the first stamps produced in Hawaii and were used primarily in correspondence between missionaries, hence their name. 
6. Inverted dendermonde
Dendermonde, which features Dendermonde town hall in reverse, is Belgium's biggest contribution to misprints in the world of stamps. Although this error was publicized through two flyers from the first printing and a flap from the second, only 17 are known. It is said that two of these stamps were lost when a famous stamp collector was murdered in 1942. If you could get hold of the few remaining stamps, it would bring in about R$300,000.
5. Inverted Jenny
Another misprint is what led to the huge price of the inverted Jenny. Now worth around R$3,000,000, this stamp features an upside down image of the Curtiss JN-4 airplane and was issued in the US in 1918. Only 100 copies were printed, which is why the inverted Jenny is great value. 
4. Baden 9 Kreuzer
It's not an image error that makes the Baden 9 Kreuzer an extremely valuable label, but a color error. 9-Kreuzer's seal is colored pink, while 6-Kreuzer's seals were green. However, a printing error caused many of the 9-Kreuzer stamps to be printed in green instead of pink. Only four still exist today, and one of them sold in 2008 for over 4 million reais.
3. The first two stamps of Mauritius
In 1847, in Mauritius, when it was a British colony, these stamps were created from British stamps, which bore the image of Queen Victoria. With only 26 known copies still extant, and being the first British Empire stamps to be produced outside of Great Britain, it is not surprising that Mauritian stamps fetch over R$4 million per copy.
2. Yellow Tre Skilling
The yellow Tre Skilling is considered one of the most expensive postage stamps in the world. Intended to be printed in a blue-green color, it was printed in yellow. This Swedish stamp issued in 1855 is supposedly the only copy that exists today, so it's worth over 8 million reais. The stamp has been sold more than once, with each time increasing in value, so it's best to start saving today if you want to be the next owner! 
1. One Cent Magenta from British Guiana
This stamp is the rarest and most valuable in the world, it has an estimated value of 36 million reais. A delivery of stamps from London to British Guiana in 1856 was delayed, so 1c Magenta was created in urgency and in limited numbers to ensure the continuation of trade and communication on the island. The Latin theme symbolizing the colony and the fact that there is only one made this stamp a true rarity.
Source: lefigaro.fr
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