Retired schoolteacher Richard McCaie was landscaping the garden of his Grade II listed 16th-century farmhouse at Braunton in Devon when he discovered a gold seal ring dating from 1620.
The historic ring is coming up for sale at Noonans in a sale of jewellery, watches, and objects of vertu on 14 March, with an estimate of £8,000-£12,000 ($9,580-$14,370).
Nigel Mills, consultant (Artefacts and Antiquities) at Noonans, commented: "The ring dates from 1620 and very likely belonged to Humphrey Cockeram of Cullompton in Devon. The ring bears a seal with the coat of arms of the Cockeram family and the initials H C behind.
"Humphrey was recorded as the head of the family in 1620 and lived at Hillersdon Manor in the early 17th century which is 42 miles east of where it was found. The family were great patrons of the church, and their name comes from old English 'Coccan' meaning dweller by the stream."
McCaie was very surprised by the find but only realised the importance of the ring later.
He said: "I was very excited when I dug up the ring, which has now (been) recorded on the Portable Antiquities Database."
The 71-year-old added, "We were amazed when Noonans told us the value and we are planning to use the proceeds from the sale to help our children."