Rare 50p coins: is this the least common of all?

The spare change in your pocket could contain one of the rarest coins in circulation, after the Royal Mint released a limited-run 50p piece commemorating Sir Isaac Newton.

Just 375 of these new coins were minted to mark the famous scientist's375th birthday, following an unspecified amount initially being circulated in June. It's believed to be one of the rarest coins currently in use.

The coins were released into tills last week at Newton's birthplace and former home, Woolsthorpe Manor inLincolnshire.

As with other limited edition 50p coins though, these collector's items soon started changing hands for above the coin's face value.

How much are Isaac Newton coins selling for?

The rarer the coin, the more collectors will typically pay to get their hands on it - although other factors, such as the condition of the coin, also play a key role.

While only 375 Sir Isaac Newton coins were released last week, The Royal Mint has announced plans to produce thousands more in the coming months.This is likely to bring down its perceived value among collectors, although it may remain sought-after if production is limited. One of the most valuable - and rarest - coins currently in circulation, the Kew Gardens 50p, had a production run of 210,000 pieces.

There are currently dozens of the Isaac Newton coins available to buy on eBay. Most of them are valued between £1 and £5, although there are specially certified and graded versions selling for far more.

Commemorative uncirculated editions of the Sir Isaac Newton coins were also available to buy from The Royal Mint website for £10, or directly from Woolsthorpe Manor - though at the time of writing, the Royal Mint had already sold out.

Rare 50p coins: which designs are the most valuable?

The table belowfrom coin experts Change Checker (changechecker.org) shows how rare certain 50p designs are in the British market.