After many years of trying, at the age of 82 Sir John Boardman is still looking for his American millionaire. Sir John does not know yet know his name, although he does have an old photograph of the fabulously rich American, who he hopes to one day make an exceptionally happy man.

Sir John, a retired Oxford University professor of classical art and archaeology, is, it should be said, trying to trace the mysterious figure, photographed with two guns, for the most scholarly of reasons.

For the American, thought to have had interests in Florida oil, appears to hold the key to Sir John’s five-year quest to find the lost gems of Blenheim.

For five years, Sir John has been engaged in a remarkable treasure hunt involving the 800 gems that had been assembled at Blenheim Palace by the fourth Duke of Marlborough.

The fabulous gems, from all over the world, represented the largest collection in Britain. As well as being hugely valuable, the duke’s gems were remarkable works of art, many beautifully engraved.

Historically, too, their significance can hardly be overstated, with some dating from the the third century BC, some Roman and many from Renaissance Europe.

Sadly, the hard-up seventh Duke of Marlborough chose to offer the whole collection of gems for sale at Christie’s in London in 1875, with the gems along with much of the Blenheim library having to be sacrificed in the interests of maintaining the palace. The duke’s initial valuation of £60,000 was trimmed, with the eventual reserve price put at £35,000, still the equivalent of many millions in today’s money.

They were bought by David Bromilow, who owned colleries in the North-West, not a known collector, who seems to have bought them as an investment. When his daughter eventually put them on the market in 1899, they were sold once again at Christie’s. Only this time, tragically, they were not sold as a complete collection, and the pieces were dispersed and remain scattered around the world.

That might have been the end of the story, if it had not been for an archive that Sir John was working on, which was to show the gems in an exciting new light.

The archive, containing a wealth of material relating to classical archaeology and art put together by the late Oxford Professor John Beazley, was bought for the Faculty of Classics in 1965.

Sir John was fascinated to find that it contained catalogues and notebooks relating to the gems, compiled for the fourth duke. Best of all there were casts of all the gems in the Marlborough Collection, providing researchers with precise copies of masterpieces in the collection.

The gems were shown to be hard semi-precious stones. Some were cut with intaglio figures and designs so they could be set, usually in finger rings, which were used as seals if pressed into sealing wax or clay. There were also the more familiar cameos, with figures cut in relief on layered stones. Many were more than one inch in length, some two or three inches long.

But the find only whetted his appetite to discover more about the gems and produce a definitive catalogue for posterity, containing detailed descriptions and photographs, revealing their individual stories and the hands that they had passed through.

The history of collections, for at least a century, has been a traditional subject of research at Oxford.

So, armed with the new information, Sir John and his team set themselves the formidable task of trying to trace all the 800 gems, dispersed from Malibu to Monaco and from Bari to Japan, in what became one of the world’s most intriguing treasure hunts.

For Sir John, the engraved gems are worthy of the same serious study as paintings or any other important works of art.

He said: “We have the clue to what the collection looked like from the casts we have in the archive. But it’s hard to understand them properly until you see a photograph. It is a very open-ended search and likely to remain so for a long time.

“The task has been not only to study and describe the gems, many of them ancient, and going back to the Renaissance collection of the Gonzago Dukes of Mantua, as well as later sources, but to try to find out where they are now.”

So far he has managed to locate about 260 of the 800 gems. “More are being found month by month and there are many more out there still to be identified and published.”

Which brings us back to the American millionaire, whose identity could hold the key to finding the whereabouts of hundreds of the gems. For, in 1920, this mysterious and impressive businessman is known to have bought some 300 gems that had once been in the Marlborough Collection from Mosheh Oved, a dealer in London.

We know a good deal about him because Oved provided a detailed account of his dealings with the American, who clearly fascinated him, in his memoir, which has survived. The dealer refers to him as ‘Superman’ but maddeningly does not name him, perhaps reflecting the time-honoured discretion art dealers maintain about buyers and sellers.

Oved recalled how the American walked into his shop and bought a box of 300 rings set with gems, most of which were from the Marlborough Collection. The next day he returned “dressed in a dirty black straw hat” and bought more gems and the Moroccan box in which had been kept at Blenheim Palace.

Four years later, he returned and admitted that the items were still unpacked in his cellar as he had lost interest in gem collecting.

But he continued regularly to buy other items from Oved, revealing details about himself.

He had apparently started at 30 as a newspaper seller and had gone on to become a director of 100 companies, often “helping in Europe”.

He had addresses in San Francisco and Switzerland, stocks in Canada and an interest in Florida oil. He learned languages, was a vegetarian and something of a health freak, and rented a luxurious house in London. He even sent a photograph of himself in fancy dress to Oved.

The Oxford professor has made appeals for information in the United States and is now focusing his efforts on the American business community. I have no doubt that he will be identified eventually and perhaps we will then be able to trace what happened to the gems through his heirs.

“It may turn out that gems worth tens of millions may still be lying in a box in a cellar or they may even have been thrown out, reckons the professor.

“Then again they could be locked up in the vault of a Swiss bank, where they have been forgotten about.”

It’s a mind-boggling prospect, given that one of the latest gems to have been recognised from the collection, owned by Yves St Laurent, was sold in Paris for 97,000 euros.

Other gems have come to light in all kinds of ways. A cameo of a dog that was published online by the Beazley Archive was Googled by its owner, and as a result rediscovered.

Five gems long ago found their way back to Blenheim Palace, probably acquired by or for the ninth duke, and are still exhibited at the palace.

The Felix Gem, is now in the Ashmolean Museum, Oxford, having passed through the hands of Captain E.G. Spencer Churchill and Sir Arthur Evans, the great excavator of Knossos and a collector of ancient gems.

This gem, like about a third of the Marlborough Collection, can be traced to the great 17th-century collector Lord Arundel. It seems that when the famous Mantuan collection became prey to dealers, Charles I snapped up the paintings, while Arundel acquired the gems.

The Arundel collection, however, only represented about a third of what the fourth duke was to amass at Blenheim, where the gems were kept in ten Morocco bound boxes.

Sir John, long recognised as Britain’s most distinguished art historian of the classical world, clearly admires the fourth duke.

In a line of heroes, rogues, spendthrifts and eccentrics, he was a man of good taste, who also had a keen interest in astronomy.

While the collection undoubtedly brought prestige — the duke proudly holds a cameo in the famous Reynolds portrait of his family — it seems that they gave him huge personal pleasure. It is said he kept them in his bedroom, where he would spend hours admiring them.

Sir John has plenty of time to contemplate just what was lost to Blenheim and Britain. He lives in Woodstock, only a short distance from the palace. With the project still far from finished, he decided the time had come to set out the story so far in a richly illustrated book The Marlborough Gems, published by the OUP, which includes an impressive, if incomplete, catalogue.

“I still get excited every time a new identification is made,” said the professor. “Generally, people make identifications from what they see on our website, but we would go anywhere to study and photograph them. Maybe one day the whole collection will be accounted for.”