The group huddled together in the farmer’s field, staring at the dozen or so medieval silver coins. They had found one, then two, and a third — Adam Staples knew, then, this was something special.

Mr. Staples had gathered with six friends that day in 2019 to try out a new metal detector, part of a hobby in which he had indulged for years. But everyone realized, looking down at the pieces of old metal: This find could change their lives.

Everyone, that is, except the farmer.

“I was jibbering on about the Battle of Hastings,” said Mr. Staples, of the entertaining exchange between him and the landowner. At the time, the farmer didn’t believe the fragile metal pieces were even coins. Unconvinced, the man fetched an old bucket for the wide-eyed hunters. Crack on, he said.

Five years later, the hoard of coins — known as the Chew Valley hoard, for the region of Somerset, England, in which they were found — has been confirmed as the most valuable treasure ever discovered in Britain. It was acquired by the South West Heritage Trust, an independent charity, for 4.3 million British pounds, or more than $5 million, which will be split among Mr. Staples, his friends, and the farmer.

Mr. Staples, a longtime user of metal detectors whose hobby led him to become an auctioneer in rare coins and other antiquities, knew as soon as they had discovered the second coin that the group had stumbled on something significant.