It's the first time that the Royal Mint has bought into a coin dealer; and it's refusing to give any details, saying only that it will in future be able to offer 'a wide range of historic coins across all periods of British coin production'.
Sovereign Rarities mostly values and sells 19th and 20th century coins, although it does handle historic coins too.
"We have a great deal of experience in both researching and cataloguing and have been responsible for a number of world record prices," says the firm.
"We are always keen to buy either single numismatic items or whole collections, and are able to offer direct purchase, auction, or private treaty."
Taking a stake in the firm is a clever move for the Mint, allowing it to take some of the profit from sales of coins which it generally produced in the first place.
"The collaboration with Sovereign Rarities Limited supports the strategic direction in which The Royal Mint is keen to progress," Anne Jessopp, director of consumer coin at the Royal Mint, tells the Sun.
"Both our businesses will be working to accelerate opportunities within this specialist area."
There are currently plenty of places to get coins valued and sold, but none with quite the history of the Mint itself. If you think you may have a rarity, your first point of call might well be a website like ChangeChecker or Check Your Change.
And over the last year, we've listed some of the rarities and errors that can make the coin in your pocket worth way more than its face value. There's a list of some of the more valuable 'error' coins here, for example, with the most valuable £2 coins here and £1 coins here.
But be warned: coins are usually only valuable to collectors if they're in perfect condition - and many wrong-looking coins are simply fakes.
Incredibly valuable coins
Incredibly valuable coins
This Australian coin was the first half crown minted under Edward VII. The price for a Melbourne coin in good condition is particularly high because around half of them were produced with faults. It’s now worth £7,500 and has risen in value some 13,789% since it was first in production
The only half crown on the list gets its position from its rarity value. However, the fact this is a silver coin rather than a gold one does affect its value - so it’s worth £10,500. It’s significantly less than others on the list - but it has still appreciated 79,445%.
This is the newest coin in the top ten, and the first year that sovereigns were produced featuring the Queen. The coin was produced in small numbers for investors - rather than for circulation - so is thought to be worth £12,500, due to its rarity.
This is another collectable gold coin prized for its rarity value. It’s worth £15,000 today and has appreciated 191,716%
This was issued in very small numbers, as it was produced during WWI. As a result, few are available - especially as uncirculated coins - so one in good condition will fetch £16,000.
This is another coin prized for its rarity, thanks to a relatively low number being minted, and more being taken out of circulation during WWI. It’s now thought to be worth £17,000 after appreciation of 42,084%.
This 1926 coin has shot up in value and is now worth £31,500. The rise in value is partly to do with a very low mintage, and partly to do with the fact that people were asked to hand their sovereigns over to be melted down during WWI, which took many of them out of circulation.
This brass threepence from 1937 has benefited enormously from the fact that Edward didn’t stick around for long to get too many coins struck in his image before he abdicated. It is now worth £45,000.
This 1933 penny has seen a stunning appreciation in value and is valued at a whopping £72,000 today. The value is due entirely to rarity. Only around seven British versions of this coin were minted, and were intended for the King to bury under the foundation stones of new buildings. They have been subject to theft, and a few are said to be in private hands now.
This isn’t the oldest coin in the list, but it was produced in a year when all gold coins were recalled and exchanged for paper money - so the vast majority were melted down. Its rarity and popularity puts it head and shoulders above the rest. It is worth an eye-watering £6,500,000, and has increased in value 2,178,885% since it was produced.