Metal detectorist Buffy Bailey, 48, unearthed the piece which is just 1.5cm long while searching farmland near York with her husband Ian.
Experts believe it may date back to the 1480s reign of Richard III and is similar to the Middleham Jewel, a 15th century gold pendant that sold for £2.5million.
Mrs Bailey, an NHS nurse from Lancaster, said: “It was so heavy and shiny – just absolutely beautiful.
“I dug down five inches and it was just there – I still didn’t believe it was anything special.”
But after she cleaned the item, she realised she had found something unique.
The engraved marker, shaped like a book, weighs just 5g and is either 22 or 24ct gold.
“Whoever had it commissioned must have been incredibly wealthy,” Mrs Bailey added.
“There’s nothing else like it in the world. It could be worth £100,000 or more.”
Mrs Bailey said she and her husband had chosen York because “it had a lot of history” and she found the object almost as soon as she arrived.
The gold bookmark is engraved with images of St Leonard and St Margaret, patron saints of childbirth, and could have been an object used for protection during pregnancy and childbirth.
As it was found near a property once owned by Richard III (1483 to 1485), experts say it could have been owned by a female relative of his or of his wife Anne Neville.
Julian Evan-Hart, editor of Treasure Hunting magazine, said the book was an “exceptionally unique” historical artefact.
“The artwork is clearly iconographic and bears a close resemblance to the Middleham Jewel – there is every possibility that it was made by the same artist,” he said.
The Yorkshire Museum, in York, is assessing the item before an auctioneer sets a valuation. At that point the museum may decide to buy the item.
The museum paid £2.5m to acquire the Middleham Jewel in 1992.