Jamille Swainson, 31, hid the sapphire and diamond studded ring in the ground before taking Harriet Haseler, 30, on an 'activities day' which included a session using metal detectors.
The excited Haseler detected an 18th-century penny, a musket ball, a silver sixpence and a silver Victorian locket before coming across a metal box shaped like a military drum. When she opened it, there was a hessian sack inside where the ring was being hidden.
"I asked her to have a look inside the hessian sack and then went down on one knee," Swainson said. "She was speechless."
Regaining her voice, Haseler agreed to marry Swainson, whom she had dated for two years after the couple met on Tinder.
"We had been finding things all morning, so I thought it could be some kind of penny," Haseler said. "I then found an old tin which opened, and inside I found an old bag with this gorgeous shiny new ring inside.
"I was speechless and couldn't stop smiling, I was beaming. It was the most amazing feeling and I couldn't believe how sweet the proposal was."
Swainson hit on the unusual proposal idea after Haseler told Swainson of her fond memories of going metal-detecting with her grandmother on a farm in Buckinghamshire.
Swainson said: "Harriet had mentioned a few times that she wanted to go metal detecting as a family so I thought it would be a great way to propose.
"A big proposal didn't interest me at first but my relationship with Harriet continued to surprise me and she changed my mind without even knowing it," Swainson said. "I started planning it in August and got in touch with Chris Langston, who runs the metal detecting company."
After asking permission from Haseler's father and seeking advice from her twin sister Scarlett, Swainson set to work on finding the perfect ring.
"She never wears jewelry so I had nothing to base it on, but Scarlett told me that Harriet liked sapphires," he said. "I had the ring made … and I had the pleasure of hand-picking the individual stones, band and setting of the ring."
After careful planning, Swainson settled on a ring with a 1.5-carat sapphire surrounded by a cluster of diamonds on a yellow gold band also covered in diamonds.
He then organized the proposal to fall on the weekend of Haseler's 30th birthday.
"I planned a surprise weekend away for her birthday, she didn't even know where we were going until the week before. I told her that I had some activities planned for the weekend, but she didn't work out that we were going metal-detecting until she saw the detectors in the back of the van."
Swainson, a builder from Buckinghamshire, England, had given the engagement ring to Langston the night before, so that it could be hidden. The couple then went with Langston to a nearby paddock in Shropshire, where they were joined by a horse named Crunchy as they spent two hours metal-detecting.
Swainson is now planning to fix the silver locket that Haseler found, so that she will be able to wear it as a reminder of the happy day.
"It's really nice that this has all happened and Harriet will never forget our special day," he said. "I kind of feel like I nailed it!"
The couple has already started planning for the big day, deciding that they don't want a formal wedding because they both have such a passion for the outdoors.