Mary Hannaby, 57, said she and her son, Michael, 33, were walking in a Hertfordshire field when they discovered the artifact, believed to be part of a reliquary or pendant, 4 inches below the ground's surface, where experts said it had likely lain for about 500 years, The Daily Telegraph reported Thursday.
"You can literally miss things by inches," Michael Hannaby said. "We couldn't believe it. We always dreamed of finding treasure."
Roger Bland, head of treasure at the British Museum, said the item, which depicts the Holy Trinity, is an "important find" but the museum does not have the money to participate in its auction, scheduled for July 9 at Sotheby's in London.