Of all the strange and surprising objects to have survived from antiquity, one of the most puzzling is the fragile Portland Vase in the British Museum. Since it was first recorded at the turn of the 17th century in Rome, where it was supposedly discovered in a sarcophagus in the tomb of the emperor Alexander Severus, this exquisitely worked example of imperial Roman cameo glass has resisted the ingenious efforts of generations of art historians to understand the mysterious scenes carved upon its sides.

Over the centuries, scholars have suggested up to 50 different interpretations explaining the imagery on the vase. Of the seven figures carved out of opaque white glass overlaying a deep and translucent cobalt-blue background, the identity of only one character is certain – that of Cupid. Holding a bow and a torch, and fluttering in mid-air, he turns back as though to entice a heroic young man towards a seated, semi-naked woman embracing a slithery monster, possibly a sea serpent, rearing up between her legs.

The identity of the rest of the group is up for grabs. Does each side represent a different mythological wedding – of Peleus and Thetis, and Achilles and Helen? Perhaps the figures represent historical personages such as Mark Antony and Cleopatra? Or maybe both scenes celebrate the first emperor of Rome Augustus, who is sometimes suggested as the vase’s original owner? The truth is that scholars will probably never agree on the meaning of the Portland Vase, which is now enshrined as a conundrum of the classical world more fiendish than the riddle of the Sphinx. Solving its perplexity would diminish its mystique.

A cameo role

The Portland Vase is the most famous example of ancient cameo glass, a type of luxurious vessel inspired by intricate relief-cut gems that thrived in the early Roman Empire up to around AD 50 or 60. Very few examples of imperial Roman cameo glass survive – in 1990 the scholar David Whitehouse counted 15 major extant vessels and objects, including the British Museum’s Auldjo Jug and the Blue Vase from Pompeii that is now in the Naples Archaeological Museum. This may be partly ascribed to the time-consuming difficulty of producing cameo glass: it took John Northwood three years to produce the first glass replica of the Portland Vase in 1876. Unearthing a vessel as fine as the Portland Vase, then, is exceptionally rare.

So when Bonhams auction house in London released photographs of a privately owned Roman cameo glass vessel four years ago, it was a big deal. The Portland Vase, which is 24.5cm (10in) high and weighs 1.3kg (3lb), is decorated with seven figures. The new vase, which stands 35.5cm (14in) tall and weighs 2.85kg (6lb), and is also made in dark blue glass with an opaque white overlay, boasts 38. These are divided between two dense friezes including a scene that depicts the punishment of the mythological character Dirce, who was killed by being tied to the horns of a bull. The lower frieze, which presents a battle scene containing 18 figures including five on horseback and five corpses, provides a clue to the original shape of the Portland Vase, which most likely was not flat-bottomed in antiquity, but tapered like an amphora.

“It is the single most important piece of glass to appear since the Portland Vase,” explains the glassmaker David Hill, one of seven people invited to the British Museum to inspect the object alongside the Portland Vase and the Auldjo Jug. “It’s as if the matching pair to Michelangelo’s David suddenly turned up out of nowhere – only this time, it’s Goliath. It dwarfs the Portland Vase.”

“It is immensely important,” agrees Martine Newby, a Fellow of the Society of Antiquaries of London who, at the invitation of Bonhams, was the first person to inspect the vase in 2009, reporting her findings in the Glass News journal the following year. “It is indescribable, really – a tour de force. It would be a masterpiece of ancient art of whatever material. It doesn’t matter if it was silver or gold or some of the best frescoes, this would have been one of the most expensive objects that you could possibly have.”

What would the vase have been used for? “It was presumably a cinerary urn containing the cremated remains of an incredibly important, high-ranking individual,” Newby explains. “It was either a personal imperial gift, or for somebody who could afford one of the most luxurious pieces in existence.”

So where is this masterpiece of ancient art today – and why is it not on display in one of the world’s great museums? According to Bonhams, who say that it was never for sale, the vase, which is broken, was consigned for study, restoration and “public exposure” before being returned to its owner. Its provenance is unknown, but, according to experts, the consignor was the daughter of a dead European collector, who was given the vase by an Italian friend soon after the Second World War.

Restoration drama

However, several experts I have spoken to suggest that the story behind the vase is murkier and more complicated. After the vase had been studied at the British Museum, there were plans to send it to Cardiff University for scientific analysis. Before it could get there, though, rumours began to emerge that it had been discovered recently in North Africa, and that it was broken when it was found. It even appeared that the vase had been damaged permanently within the past decade – a botched attempt at restoration, perhaps, or, more sinisterly, a sneaky way of passing off a recent restoration as something older, so that the vase would appear authentic. With its provenance in question (for instance, nobody could prove the existence of the shadowy Italian, supposedly a well-to-do family friend who had given it to the consignor’s father), the vase, which could be worth millions of pounds at auction, was quietly returned to its owner. “It was really disappointing,” recalls Newby, “because there I was working on it, and then it was taken away from me, and that was it – the doors were closed.”

The vase now languishes in unsellable limbo. Its whereabouts are unknown, although amongst experts there is a whisper that it is hidden away in a bank vault in Brussels, denied the recognition of its celebrated cousin, the Portland Vase.

Of course, the antiquities market is plagued with fakes, and not everybody believes that the vase is ancient. Paul Roberts of the British Museum, who studied the vessel alongside the Portland Vase and the Auldjo Jug, declined to comment. But Newby and Hill are both adamant that it is the real deal. “No faker could have included details that are only known to the tiny handful of glass anoraks like us who have seen these things in pieces and understand how they were made,” says Hill. “Even the very best fakers in glass are woefully poor craftsmen, and any attempts at faking Roman cameo glass are doomed from the start. The bottom line is that the glass itself needs to be tested. When both the blue and the white are proven conclusively to be Roman, there really cannot be any farther argument. The vase is a once-in-a-lifetime discovery, and we just want to see it treated with the respect it deserves, for few of us are likely ever to see anything as important as this appear again.”

Alastair Sooke is art critic for The Daily Telegraph

If you would like to comment on this story or anything else you have seen on BBC Culture, head over to our Facebook page or message us on Twitter.