The small pearl brooch in Raphael's masterpiece, La Fornarina, was the clue, and researchers believe that it has unlocked one of the mysteries of the Renaissance.
For centuries it has been thought that the woman, Margherita Luti, a baker's daughter from Siena, was the artist's mistress.
He was, after all, engaged to the niece of a powerful Vatican cardinal.
However, a new study by an Italian art historian, Maurizio Bernardelli Curuz, suggests that Raphael and Luti had already married in a clandestine ceremony, and that - fearful of a scandal - the artist took the secret to his grave.
Officially, he died a bachelor at the age of 37.
"It was an impossible love affair," said Mr Bernardelli Curuz. "It is hard to overstate Raphael's status in Rome. He was a superstar. The distance separating them was like that which today would separate George Clooney and his cleaner."
La Fornarina is one of the painter's most suggestive portraits.
Half-clothed and with a coy expression that has been likened to the Mona Lisa, the dark-eyed beauty has a diaphanous veil over her stomach and suggestively cups her left breast.
Mr Bernardelli Curuz, editor of Stile magazine in Milan, which conducted a year's research into the romantic riddle, says a string of nuptial allegories in the portrait point to the secret marriage.
The biggest clue, he says, is the brooch pinned on to the fashionable silk turban of La Fornarina, the kind of expensive bauble a woman would wear on her wedding day.
The pearl, also included in another portrait of a woman, La Donna Velata, reinforces the theory that the sitter's name was Margherita - a Latin word for pearl - and not Maria Bibbiena, the artist's intended bride.
There are other matrimonial hints, according to the art expert, from the blue ribbon on the woman's arm bearing Raphael's name - an unusual way to sign a portrait - to a ring on her left hand that was later covered up by Raphael's students.
In addition, the portrait is painted with background foliage of myrtle and quince, symbols of love, fecundity and fidelity. Such allegories, says Mr Bernardelli Curuz, "may seem artificial to us, but these were everyday games at the Renaissance courts.
"At least until the 18th century, the allegorical side of painting was extremely important. It was Impressionism that dampened our ability to read a painting like a book."
La Fornarina has long fascinated admirers of Raphael's work. In 1814, the French artist Jean-Auguste-Dominique Ingres painted La Fornarina sitting on the artist's knee. A century later, Picasso portrayed their trysts in a series of explicit drawings. The novelist Honore de Balzac also mentioned the couple.
However, Raphael's students made efforts to draw a veil over Luti's existence, according to Mr Bernardelli Curuz. The painting was unfinished when the artist died and was completed by his students who covered up the bushes and the tell-tale ring. They were uncovered during a recent cleaning.
"At the time of his death, Raphael's school was painting the Sala di Constantino in the Vatican, and losing that commission could have meant bankruptcy," he said.
"He had left La Fornarina unfinished, and the students began to fret about the landscape, the ring, anything that could tie it to the marriage."
To silence the rumours, Raphael's students placed a plaque on his tomb in memory of his fiancee, Bibbiena - and Luti was sent away.
Four months after the artist's death in 1520, the convent of Sant'Apollonia in Rome's Trastevere quarter registered the arrival of "widow Margherita", daughter of a Siena baker. Mr Bernardelli Curuz said he had found evidence to support his theory from contemporary documents and x-rays of the painting.
"Of course this is not just about the pearl, nor is it just about the documents," he said. "The absolute certainty comes from the way everything fits together. But the pearl was what tipped us off."
