It's no secret that some of the rarer coins in circulation can be worth far more than their face value.
But how much a coin fetches can vary from week to week, depending on its scarcity and popularity with collectors. It means it's often hard to know whether that 50p in your pocket is worth hanging on to or not.
Now, though ChangeChecker has launched a scarcity index that tracks the rarity and collectibility of the different 50p versions and ranks them in order.
The least common (and most valuable) £1 coins
Rather than just looking at the number of coins in circulation, the site monitors how many of each version have been collected by site members, and the number of times they've been requested as a swap.
"Trying to find a good quality coin from 15 to 20 years ago, even for a higher mintage issue, is much more challenging than a more recent issue, as coins become damaged over time and are ultimately removed from circulation," explains ChangeChecker's Yasmin Britton.
"Additionally, some designs are more hoarded than others by people who might not normally collect coins – the Beatrix Potter issues being a classic example. Finally, it can be up to a couple of years before the Royal Mint eventually confirms the actual mintage for an issue."
Top of the list, as ever, is the Kew Gardens 50p, which can change hands for hundreds of pounds when in good condition. Only 210,000 were ever minted, and it's the least-owned and most requested-swap on the ChangeChecker site.
The next four scarcest coins are all from the 2012 Olympics – Football, Triathlon, Judo and Wrestling - but there are plenty of others that are worth more than their face value.
"The recent Beatrix Potter 50p coins also rank highly, with Jemima Puddle-Duck and Squirrel Nutkin coming in at numbers 6 and 7 – based largely on their low level of ownership and high swap requests, as official mintages are yet to be confirmed by the Royal Mint," says Britton.
These tend to be worth a couple of quid each.
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The scarcity index will be updated every quarter, says ChangeChecker, as new coins are released and the popularity of different versions rises and falls. Here's the current top 20, with their Scarcity Index score.
1. Kew Gardens: 100
2. Football: 78
3. Triathlon: 76
4. Judo: 74
5. Wrestling: 73
6. Jemima Puddleduck: 61
7. Squirrel Nutkin: 56
8. Tennis: 56
9. Goalball: 54
10. Handball: 46
11. Shooting: 44
12. Beatrix Potter: 40
13. Taekwondo: 40
14. Mrs Tiggywinkle: 39
15. Gymnastics: 38
16. Hockery: 35
17. Pentathlon: 32
18. Sailing: 32
19. Rowing: 30
20. Peter Rabbit: 27