The unique Diamanti exhibition, due to end its four-month run at the Scuderie Papali, opposite Palazzo del Quirinale, on 30 June, has turned out to be one of the revelations of this springs cultural programme in Rome. This event which organisers say may never be repeated has given visitors a once-in-a-lifetime opportunity to see many of the worlds costliest and most famous diamonds in their historical context, displayed alongside portraits of some of their past owners. Exhibits many of which have come from private collections and are being seen by the public for the first time come from all over the world.

However, the most incredible thing about this show is the fact that it was born almost by chance. We were preparing a major exhibition on Leonardo da Vinci when the Twin Towers were attacked last September, recounts Caterina Cardona, the dynamic head of the former royal stables exhibition organisation department. The immediate reaction of American museums was to block all loans. I believe that this tragedy, in fact, is going to have far-reaching effects on all future international exhibitions all over the world. For one thing, art gallery and museum curators have become very reluctant to let their possessions out of their custody and, secondly, the cost of insurance has rocketed up to three or four times more than before.

Since many of the works due to be shown in the Leonardo exhibition were in the United States, Cardona and her team were forced to postpone the show indefinitely. Meanwhile, they urgently needed to fill the gap.

We heard of an exhibition which was running in Paris at the Muse National dHistoire Naturelle. Mainly, it was about diamonds, but, in addition to stones, there were one or two paintings. The idea for this exhibition took off from there.

With the help of leading gemologist Hubert Bari of the Paris museum and Maria Sframeli from the Florence, Pisa and Pistoia department of fine arts and historic heritage, Diamonds in Art, History and Science was put together in record time. On 1 March the doors opened on a stunning display of some of the worlds finest jewels and gemstones, offset by a series of paintings by Titian, Rubens, Botticelli, Ingres, Balla, De Chirico and Warhol.

A major exhibition normally takes five to ten years to prepare, explains Cardona. Her team, however, had already been broken in to the art of working miracles. When the Scuderie del Quirinale was inaugurated in December 1999, after the completion of Gae Aulentis ingenious restructuring, four major exhibitions were already scheduled for the following jubilee year.

Cardona admits that not only a lot of hard work but also intuition and luck went into the preparation of the Diamanti show.

We combed through catalogues to find paintings featuring people wearing diamonds, she says. The lovely Titian portrait of Laura Dianti, who became the wife of the Duke of Ferrara, belongs to a private collector. I didnt hold out much hope that he would lend it to us, but when I called him up, he agreed immediately.

Severinis I Sposi was another inspiration. I wanted something that would represent our times. I just happened to be visiting the Galleria Nazionale dArte Moderna when I spotted the diamond engagement ring on the young brides finger in the picture. It was just perfect carrying on the theme that underlies the exhibition: the diamond through the ages as a symbol of purity and eternity.

The Diamond Dust installation by the Danish artist Olafur Eliasson, which closes the exhibition, was also a chance find. It was spotted at another exhibition, and we booked it immediately.

The exhibition contains 160 diamonds and pieces of jewellery, including the De Beers and Cartier private collections. Many of the individual stones are legendary, such as the Star of South Africa, the Marie Antoinette Pink, which belonged to the ill-fated French queen, and the Power of Love, which is the biggest of the five existing red diamonds in the world. Visitors stand riveted in front of glittering marvels like the Order of the Golden Fleece, from the crown jewels of Portugal, Pope Gregory XIs tiara, studded with 17,000 diamonds, and the spectacular King of Rome necklace, presented by Napoleon to his second wife, Maria Luisa of Austria, when she produced the long-awaited heir. No less priceless and irreplaceable is the Orgeuil Meteorite, which is full of microscopic diamonds older than our own solar system. The total value of the items on display is estimated at 300 million and the most costly stone on view carries a price tag of US$42 million.

Obviously, security has been a special headache. Armed guards patrol the premises and museum personnel were given special training in spotting suspicious behaviour. A control room on the top floor monitors all movement in every corner of the building.

Those who havent yet seen Diamanti have time until 30 June, when it will close its doors. Cardona, however, is already in a flurry of preparation for another Scuderie first.

Our next major international exhibition will be La Maest di Roma, scheduled for next spring, Cardona reveals. Our partners this time will be the Galleria Nazionale dArte Moderna and the French Academy at Villa Medici. It covers the golden age between Napoleon and the unification of Italy, when artists from all over Europe flocked to Rome to work and its going to be absolutely fascinating.

Diamanti. Until 30 June 2002. Scuderie Papali al Quirinale,

Via XXIV Maggio, tel. 06697270. 10.00-20.00.