It is just one penny, but it is expected to sell for more than £10,000 at an auction.
The rare silver coin from the 12th century was discovered by metal detectorist Graeme Rushton near the site of the 1141 Battle of Lincoln. It depicts Stephen, King of England from 1135 to 1154, and his wife Matilda.
The 50-year-old had been searching a field near Lincolnshire’s border with South Yorkshire and dug down about 6ins before uncovering the Stephen and Matilda penny.
The Stephen and Matilda penny – an extremely rare 12th century silver penny which is tipped to sell for £10,000 after being discovered in a ploughed field by a metal detectorist
At first Mr Rushton, who owns a metal detecting shop, did not recognise the coin, so he took it home for safekeeping.
It was only a year later when he decided to contact the Fitzwilliam Museum in Cambridge about it that he became aware of the significance of his discovery.
Mr Rushton said: ‘It was only my second visit to the site which had just been ploughed and flattened.
‘After about 45 minutes walking up a slight rise in the field, I got a signal, and after digging down 5 to 6 inches, I uncovered the coin which at first I didn’t recognise.
‘It was only after showing pictures of it to the Fitzwilliam Museum in Cambridge that I realised how significant the discovery was.
‘It is quite exciting. I will share the money with the landowner and I’ve not really thought about what I’d do with the rest.
‘For me, the most interesting thing is uncovering a piece of history. I’ve been detecting for 40 years and that still gives me a buzz.’
The coin, which was minted in York in the early 1140s, has standing figures of Stephen and Matilda facing each other supporting a tall spectre between them.
One side of the coin, which Graeme Rushton, 55, did not recognise at first but took home for safekeeping and decided to contact the Fitzwilliam Museum in Cambridge a year later
For almost all of his reign Stephen fought against his cousin, another Matilda, who had been the nominated heir to the throne.
The English Royalist barons supported Stephen while the Angevin French supporters backed Matilda.
The coin was found was not far from where the Battle of Lincoln took place in February 1141 between Stephen and Matilda’s half brother Robert, Earl of Gloucester.
Stephen was defeated, captured and imprisoned for six months in Bristol Castle before an exchange enabled him to be released.
During this time, his wife Matilda, a very powerful woman in her own right, actively campaigned for him to be let go.
The other side of the coin, showing standing figures of Stephen and Matilda facing each other supporting a tall spectre between them
The civil war ended in 1153 when Stephen accepted Matilda’s son Henry as his heir. He became Henry II, founder of the Plantagenet dynasty which ruled until 1485.
Graeme is now selling the coin, which is thought to be one of only 25 in existence, with London-based auctioneers Dix Noonan Webb.
Nigel Mills, antiquities specialist at Dix Noonan Webb, said it was ‘special’ to find this type of coin still intact and not badly damaged.
He said: ‘There is some contention about whether it is Stephen’s wife Queen Matilda or his cousin Matilda who is on the coin with him.
‘However, we believe the person is his wife Matilda as it would be unusual for him to choose to have the picture of a cousin he was fighting with on there.
‘The Queen Matilda actively campaigned for his release when he was imprisoned in Bristol Castle after the Battle of Lincoln.
‘She was a very wealthy lady and Stephen’s marriage to her made him very powerful. There are an estimated 25 examples of this coin in existence but a lot of them are badly damaged and fragmented.
‘The fact that this one is intact makes it quite special. It was minted in York in a very continental style so likely by a foreign engraver.’
The sale takes place on September 16.