Mark Thompson, who spray paints fork lift trucks, was in the famous Nottinghamshire woodland for just 20 minutes when his metal detector sounded.
The glint of gold caught his attention and he could hardly believe what he unearthed.
The ring, which is engraved with an infant Christ on one side and a female saint on the other, is believed to date from the 14th century.
The 34-year-old, who had only taken up the hobby 18 months ago, was expecting to find something innocuous.
But as he shovelled the dirt, a glint of gold emerged on ornate piece of jewellery with a precious sapphire.
Auctioneers have since told him that the precious ring could fetch between £20,000 and £70,000.
Mr Thompson said: “I had been out metal detecting with a group for about 20 minutes when I heard the signal.
“I was really excited when I saw that it was gold, but I didn’t realise at that point just how significant it might be.