The ring is expected to draw bids ranging from £15,000 (around $20,096.55) to £20,000 (around $26,792) when it goes under the hammer at Noonans of Mayfair on June 23.
“I know I may never find anything that surpasses this discovery, but that is part of what makes it so special,” Stuart said.
As he carefully pulled the ring from the soil, one diamond came loose and fell into his hand, while a second stone was also found to be missing.
He collected the surrounding soil from the discovery site, then washed and sieved it, successfully recovering the missing diamond.
The proceeds from the auction will be shared equally with the landowner. The ring has also been examined by the British Museum.
Laura Smith, a jewellery specialist at Noonans, said: “Early 17th-century baroque taste required grand rings to make an impression from a distance.”




