Is it or isn’t it? If it is, it is probably worth more than $90 million. If it isn’t, it won’t fetch one-thousandth of that price. The factor that makes the colossal difference is simple: the name “Michelangelo.” The hand of the great Florentine may have been responsible for carving a marble relief exhibited for just two days last month in the Tuscan town of Vinci.

I was lucky enough to see the sculpture for myself during its very brief stint in the Museo Ideale, a charming private museum. The sculpture itself is a tondo, or circle, about 14 inches in diameter. At its center is a bearded head depicted in three-quarter profile.

It is an extremely beautiful work, carved with precision, elegance, and delicacy. But did it come from the hand of the master? James Beck, a professor of the history of art at Columbia University and a Michelangelo scholar, believes it could have.

In 1999, Mr. Beck published a monograph on Michelangelo, “Three Worlds of Michelangelo” (Norton), in which he identified the piece as a possible work by the master. Since then, there has been a slow but steady increase in interest in the tondo, led by Alessandro Vezzosi, the director of the Vinci museum. As a result, the owners finally decided to exhibit it, albeit briefly.

If the tondo is, indeed, by Michelangelo, it would be a sensational discovery. No new sculpture by the artist has appeared on the art market for at least 100 years. Drawings by him have surfaced in recent decades (Christie’s sold one for almost $14 million in July 2000), but none of the marble carvings attributed to him in recent years has survived scholarly scrutiny.

“The vast majority of works which have been put forward as ‘new’ Michelangelos,” said Mr. Beck, “have been easy to rule out. They just aren’t technically or aesthetically competent enough to be by so great a genius. But this is a work of exceptionally high quality. There’s no question that it is good enough to be by Michelangelo.”

The skill of the carver is evident from the expression on the old man’s face, which combines the appearance of awed contemplation of the infinite with a hint of contempt for the quotidian world of the merely mortal. It is not unlike the expression found on Michelangelo’s figure of Nicodemus, part of his late Piet, now in the Museo del Opera del Duomo in Florence. That figure, too, is thought to be a self-portrait: It certainly has Michelangelo’s features – although, according to Ascanio Condivi, Michelangelo’s assistant and biographer, the artist was so frustrated by his inability to convey his vision in stone that he tried to smash the whole ensemble.

Michelangelo, who lived from 1475 until 1564, worked on the Florentine Piet between 1547 and 1555. Mr. Beck dates the marble tondo to between 1545 and 1555, when Michelangelo would have been in his 70s. “It is stylistically clearly from that period,” he said.

“On the other hand,” he cautioned, “there’s no provenance for the tondo: There are no contemporary documents, there is nothing which suggests that Michelangelo carved such a work in either Giorgio Vasari [who also wrote a biography of Michelangelo during his lifetime] or Condivi. There is an 18th-century document from a palazzo near Pisa, which describes a marble relief as a ‘self-portrait by Michelangelo’ … But we don’t know that the document refers to this tondo – and, even if it does, it certainly doesn’t prove that this tondo is by Michelangelo; it just proves that, in the 18th century, the people who owned it thought it was by Michelangelo.

“Of course,” Mr. Beck added, “none of this proves the piece isn’t by Michelangelo – and I believe that it may well be. But I can’t prove it.”

Mr. Beck’s possible attribution – tentative and qualified though it is – is nevertheless a very unusual claim for him to make. He is famous, or perhaps notorious, for pouring cold water on claims that works are by Michelangelo. He has written, for example, a comprehensive demolition of the claim that a stone cupid – known as the “Fifth Avenue Cupid,” because it is held by the French Cultural Services Institute on Fifth Avenue in Manhattan – is by the master sculptor.

“Whenever the name ‘Michelangelo’ is mentioned,” Mr. Beck said, “people go a little crazy. It’s the money. There’s this constant urge for scholars and others to persuade themselves they have found one. But almost invariably, despite all the hype and all the excitement, it turns out that what is initially claimed as by Michelangelo isn’t by him at all.”

What makes him believe that there is a chance the marble tondo will be any different? “The quality of carving.” The features depicted in the tondo relief are certainly those of Michelangelo. They are strikingly similar to those depicted in the best portrait of Michelangelo – it was painted while Michelangelo was still alive – by Jacopo del Conte. If Michelangelo was the sculptor, the tondo would be a self-portrait in stone: a fascinating glimpse of how Michelangelo saw himself in old age.

Stylistic considerations alone will not persuade skeptics, however. They will want scientific evidence – at least for a date that would put the sculpture’s creation definitively in Michelangelo’s lifetime. Unfortunately, scientific data on that issue seems to be unobtainable.

It is possible to date accurately oil paintings from the 17th century from the wood on which they were painted, and even, at least occasionally, from the pigments in the paints themselves. But with stone precise dating is impossible. The marble is about 50 million years old. As Corrado Gratziu, a professor of geology at the University of Pisa, explained to me, “The greatest accuracy you could expect would be to date the marble to within 100,000 years of when it was first formed – which is, of course, no use whatever in trying to date when someone started carving it.”

Mr. Gratziu has discovered, however, that the tondo’s marble came from Carrara in northwest Tuscany. Even more precisely, he has ascertained that it came from the Polvaccio quarry at Carrara, which is known to have been used by Michelangelo: Even today, it is known as “Michelangelo’s quarry.”

Mr. Gratziu has also found that the relief had been left out in the open for a very long period – perhaps as long as 150 years – and cleaned with acid, traces of which still survive on the surface. “The use of acid,” he explained, “together with the long period the relief spent outside, explains why its surface is so smooth.”

It is a discovery that aids the attribution to Michelangelo. “The one thing that bothered me,” Mr. Beck said, “was the smoothness of the carving. That is not characteristic of the works in stone of Michelangelo’s maturity, when he was famous for the ‘non finito,’ or ‘unfinished,’ look that he gave to his sculptures. The fact that the smoothness in the tondo was not the result of the artist, but of cleaning acid and of it having been left open to the elements, makes it consistent with Michelangelo’s late style.”

The tondo’s present owners bought it cheaply 30 years ago. They are keen to remain anonymous, and to keep secret the place where they store the work. Art theft is very big business, and an easily transportable stone sculpture that could be worth so much is an irresistible magnet to every art thief in Italy, and possibly the world.

Mr. Beck thinks he has a solution to the present owners’ problem of keeping the tondo safe. “I think they should give the tondo to an Italian museum,” he said. “That way, the issue of who sculpted it could be decided … without auction houses, or academic experts, having a financial interest in attributing the piece to Michelangelo. It would be a lot easier to get at the truth.”

The sculpture’s present owners, however, do not seem to be very keen to take up that suggestion. With so much money at stake, you can see why.