If you think stamp collecting is just for hobbyists and not something a shrewd investor would consider, you may want to think again. Mint condition specimens have appreciated by up to 45.5% over the past 10 years, according to analysis by StampFinder for Forbes, easily beating typical returns on real estate, gold, fine wine and the broad stock market. And the rarest philatelic treasures can sell for millions, with world record prices being set recently. Read on to see the most valuable stamps of all time.

Bermuda's very first stamp is prized among collectors. Just 11 specimens exist and three of them form part of Queen Elizabeth II's Royal Philatelic Collection. This beauty was sold by London-based stamp collecting business Stanley Gibbons in 2016.

This rare Penny Black proof has a striking over-inked appearance that adds hugely to its value. Only three examples of this sort of proof remain and one of them is part of the Royal Philatelic Collection.

High-denomination UK stamps are few and far between. This 'One Pound' brown-lilac coloured stamp dates from 1882 and is one of just four specimens, of which it is apparently the finest. It was sold via Stanley Gibbons several years ago.

Another fortuitous accident, in 1864 printers produced 54 sheets of Hong Kong 96¢ stamps in the wrong colour – olive instead of brownish grey. The 40 stamps that remain command immense prices at auction – a block of four sold for HK$6.4 million ($825k/£704k) in 2011.

The British Board of Education hardly ever issued stamps marked with its moniker, and this 1902 example is only one of four or five in the world. An example of this philatelic rarity was sold by Stanley Gibbons for $234,000 (£200k).

Only four examples of the ultra-scarce 2¢ Large Queen on Laid Paper are known to exist, making it the rarest Canadian postage stamp. The last example that came up for auction went for a hefty $250,000 (£213k).

A stunning stamp from the coveted 1869 US Pictorial Issue, the 24¢ Declaration of Independence stamp is highly esteemed by collectors in the know. The most valuable are unused examples that show an inverted centre – they are valued at $275,000 (£234k) a pop.

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The best-preserved unused specimens of the groundbreaking Penny Black are especially valuable. This exquisite example, which was sold via Stanley Gibbons, features rare 'T L' lettering and a fragment of the sheet inscription.

Another mistake from the 1869 Pictorial Issue that made for an incredibly valuable stamp, this inverted flags version of the 1869 30¢ Shield, Eagles and Flags stamp fetched a record $310,500 (£265k) at a Robert A. Siegel sale in New York in 2016.

An exalted American stamp, the Lost Continental is the only known example of the 24 cents General Wilfred Scott that was printed by the Continental Banknote Company – the stamp's rarity justifies its high value.

Dr Sun Yat-sen's head was printed upside down on just one sheet of $2 Hong Kong stamps in 1941. A total of 50 stamps feature the error but only two vertical pairs remain in existence – this pair realised $707,700 (£604k) at auction in 2012.

More worthy of a public museum that a private collection, this excellent example of a Frederick Heath-engraved Penny Black proof has been described as “one of the major exhibition pieces of world philately”. It was sold by Stanley Gibbons in 2016.

The daddy of Penny Blacks, Stanley Gibbons sold this master proof of the world's first stamp in 2016. Just three Penny Black master proofs are in private hands – the other three are in the Royal Philatelic Collection and the United States National Postal Museum.

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This Chinese military stamp was recalled when the authorities realised the paper was slightly transparent and there was a risk that confidential information contained within secret correspondence could be seen through the envelope. Only a few of these valuable stamps remain.

The fabled 1987 Red Revenue, aka the Red Maiden in the Green Robe, was issued as a temporary measure during currency reforms – the 2 cents face value was overprinted with green ink. Very few exist and the last one that came up for auction sold for $444,477 (£379k).

While it's not the rarest Canadian stamp, the 1851 12d Black Empress is the most valuable. Only a handful of specimens remain and the few that come on the market command colossal prices of anything up to $445,000 (£380k).

This British stamp from 1904 is considered the country's rarest. Only 19 sheets were printed before the issue was withdrawn and only three are thought to have survived to the present day. Also known as the I.R. Official, the only example that has come up for auction sold for a hefty $469,000 (£400k).

The most valuable individual stamp from the legendary collection of William H. Gross, this famed unused example of the 2¢ Hawaiian Missionary from 1851, the finest of a handful in existence, fetched $619,500 (£529k) at a Robert A. Siegel sale in New York in October 2018. It was snapped up by pharmaceuticals executive Arthur Przybyl.

An example of the rare Plate 77 Penny Red stamp was sold by Stanley Gibbons for $645,000 (£550k) to a client in Australia. Plate 77 stamps were considered poor quality and all examples were supposed to have been destroyed. The five that survived are regarded as the holy grail of British philately.

In January 2019, Spink Hong Kong auctioned this unused 1897 10¢-on-9-Candareen dark green Dragons and Shou stamp with the 10¢ surcharge inverted, from China’s Empress Dowager issue. Only three specimens of this type have ever been verified, and the stamp is the only unused example.

This exceedingly rare version of the 1867 3¢ George Washington stamp in rose is one of America's most sought-after philatelic treasures. The last example to come up at auction sold for a staggering $900,000 (£768k).

This ultra-rare stamp, the only example on blue paper which is known to exist, was issued in Alexandria, DC, way back in 1847. Its extreme rarity makes it one of the world's most valuable stamps. The stamp sold for a whopping $1.18 million (£1m) at an auction by HR Harmer in 2019.

Germany's most valuable stamp came about as a result of a colour error. The Baden 9 kreuzer stamp should have been printed in pink but green ink was used by mistake on a batch produced in 1851. Only four specimens remain and they are worth a jaw-dropping $1.5 million (£1.3m) each.

In 1918, the Curtiss JN-4 airplane was accidentally printed upside down on 100 24 cent stamps. A super-valuable error stamp, the famous 'Inverted Jenny' sells for mega bucks. In November 2018, an example sold for $1.6 million (£1.4m) at a Robert A. Siegel auction.

This Chinese stamp was recalled within a day of issue after it was discovered that Taiwan was incorrectly printed in white. One of just nine in existence, this pristine unused example went under the hammer for $2 million (£1.7m) at a China Guardian sale in Beijing in November 2018.

Much prized by collectors, the Mauritius 'Post Office' stamps were the first British Empire stamps produced outside the British Empire. Few examples exist and the last pair available on the open market sold for $4 million (£3.1m).

The 1868 Benjamin Franklin Z Grill is the rarest and most valuable of all US postage stamps. This example, which belongs to storied collector William H. Gross and is listed in the Scott Catalogue for $3 million (£2.3m), is only one of two in existence.

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This 19th-century Swedish stamp is as unique as they come – only one example is known to exist. It was printed in yellow rather than the usual blue-green, and quality control failed to pick up the error at the time. Sold for $2.3 million in Geneva in 1996, the sale price would today equate to $3.8 million (£3.2m).

China's Red Revenue stamps are renowned for fetching astronomical prices, and the rare 'Small One Dollar' series is the most sought-after. The original block of four, considered the crown jewel of Chinese philately, sold for $15.2 million in 2009. That's the equivalent of $18.3 million (£15.6m) in today's money.

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At an auction in June, a block of inverted 1918 24¢ Jenny Plate stamps with the upside-down aeroplane misprint sold for $4.9 million (£3.5m), setting a record for the sale of an American stamp. Bought by the American collector, David Rubenstein, the stamp block is set to go on public display "to help inspire and educate others about our country’s rich history".

Previously the world's most valuable stamp, there's only one known example of the British Guiana 1856 1 cent magenta left. Known as the Mona Lisa of the stamp world, and created due to a shortage of British stamps in Guiana at the time, it was sold to shoe designer Stuart Weitzman for just under $9.5 million (£8.1m) in 2014. However, Weitzman has since made a considerable loss on the stamp, selling it in June for $8.3 million (£5.9m) to British stamp collecting business Stanley Gibbons, which plans to use the acquisition to launch a new fractional ownership scheme where investors will be able to buy a share of the stamp and hopefully profit from a future increase in its value.

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