A metal detectorist kept a Roman coin for nearly 30 years before realising it was worth £10,000.
Retired police officer Tom Thomas, from Reading, made the rare discovery in the 1990s and kept the coin in his small collection.
The hobbyist only realised it was a one-of-a-kind coin when a fellow detectorist pointed out its rarity at a family barbecue.
The 62-year-old said he was “surprised and delighted” by the discovery.
Mr Thomas, who has been metal detecting for more than 30 years, had found other Roman artefacts before.
He found the coin, which dates back to 286-93AD, in a farmer’s field in Berkshire.
“I didn’t know what it was as such. I put it with my small collection and thought nothing more of it,” he said.
It turned out to be the only known example of a Carausius Denarius coin which features the Roman goddess Salus feeding a snake rising from an altar.
Friend Mark Becher, who runs a detectorists’ group in Aylesbury, Buckinghamshire, had been at a barbecue at Mr Thomas’s house.
“I’ve been metal detecting for more than 25 years and I’ve witnessed countless finds, both my own and other people’s. I’d just never seen anything like it.”
Mr Thomas is now selling the coin at auction.
He said: “The only reason I’m selling it now is because it’s so unique and valuable it has to be locked away in a bank vault.”
The rare coin will be auctioned at Hansons Auctioneers on 27 August, and has a guide price of £10,000.