Kevin Woods, who also runs a historic building restoration firm, has been detecting since childhood and was invited to Hill Top Farm, near Malham, by owners the Heseltine family, who have farmed the land for four generations.
Mr Woods' equipment unearthed the trumpet brooch - thought to date from the reign of Emperor Hadrian - just four inches beneath the surface of a stony field.
The jewel was in such excellent condition that it has been compared to similar objects on display in museums in Rome itself.
Although the find is not itself unusual and is worth little more than £60, Mr Woods has mainly come across fragments of trumpet brooches before and it is rarer to discover one intact and undamaged.
The artefact has been woven into a fascinating narrative about the Roman conquest of the Dales over 2,000 years ago, when the region was still the 'frontier' as the occupiers tried to subdue native tribes further north.
Mr Woods believes it could have been used by a Roman soldier to fasten his cloak with, as there was a marching camp large enough for an entire legion near Malham, which was used as a resting point along the route to Hadrian's Wall.
"It dates from the first century AD. It's like an ancient safety pin really, and ones this size were usually used by soldiers. It's a really nice one that's in great condition. I think because the farmer doesn't use pesticides and the soil is sandy, it has kept better.
"It's actually quite common to find them in ploughed fields, but they're normally smashed to bits."
As the brooch is not valuable, there is unlikely to be a treasure inquest, although Mr Woods has reported the find to the government and may donate it to a local museum.
Although the discovery greatly excited the Heseltine family, for Mr Woods, it is relatively small beer, and he instead sees its value in building up a bigger picture about the Roman presence in the area.
He has previously found Bronze Age spears and daggers, a Roman ball, and even an entire burial ground at Arncliffe, which then became the subject of a funded archaeological dig.
He has discovered the remains of what he believes is a Roman farmhouse and blacksmith's shop in the same valley, and found Civil War musketry when he was allowed to use his metal detector on the site of Skipton Castle.
"It's not gold and silver, but it helps to tell the story. Detecting is just a hobby for me - maybe one day I'll hit the jackpot!"
The brooch's identity was confirmed by York Museums Trust archaeologist Rebecca Griffiths, who is also the finds liaison officer for North Yorkshire.