There has never been an exhibition devoted to Russian jewels. While the fabulous Easter Eggs confected by the House of Fabergé survive, as do menageries of miniature hardstone birds and beasts, sprays galore of jewelled and enamelled flowers and any number of elegant cigarette cases, photograph frames and the like, gem-set Russian jewellery is rare and elusive.

Most of these conveniently portable assets left Russia with their humbled, exiled owners after 1917, the grander pieces invariably dismantled so that the more valuable stones could be sewn into hems and folds or swallowed. Much of what survived the revolution intact subsequently fell victim to fashion, and the stones were re-cut and reset. That London jewellers Wartski has gathered together some 260 pre-revolutionary jewels for a literally dazzling loan exhibition, "Fabergé and the Russian Jewellers" (May 10-20), is a tremendous coup and a unique opportunity. It is probable that we will not see their like again.

Loans have come from private - and some public - collections across the world. Many come from European royal and aristocratic vaults, some have been repatriated by Russian oligarchs while others, more implausibly, take a bow on regular Church of England Easter and Christmas services - one bishop's morse or cope clasp here was a gift from the Imperial family to the British Embassy for sending a discreet Anglican clergyman to exorcise a house in St Petersburg. The only American not to lend anonymously is the comedian Joan Rivers. All have generously lent their trophies in a good cause - the exhibition is in aid of The Samaritans - a tradition since Tsar Nicholas II lent his Fabergé to benefit the Society of Patriotic Ladies in 1902.

The revelation of this show is not so much the delicacy and technical virtuosity of these jewels but the intensity of colour of the Russian gemstones. It was Peter the Great who instigated the search for precious stones in an Empire as rich in resources as it was vast - so immense, in fact, that when the sun was rising on one side it was setting on the other. Catherine the Great pursued the project with equal enthusiasm in an age possessed of a mania for the natural sciences. Siberian amethysts, for instance, glory in a body colour of intense purple, and flash with fugitive highlights of red or blue. The best of them came from the Talyan mine, near Murinska, opened in 1768. One such stone here, of deepest purple shot with fiery red, was presented to Catherine and remained in the Imperial treasury until Nicholas II sold it in aid of the Russian Red Cross. Fabergé re-cut the stone and mounted it as a pendant in a diamond-set frame.

Pink topaz was another Russian speciality. The pendant here was among the jewels heaped on the Marchioness of Londonderry by an enchanted Alexander I in 1821, including a string of amethysts the size of goose eggs. The Siberian blue topaz here, weighing in at 120 carats, elicits admiration for the almost military infrastructures demanded of the court dressmakers. The pretty sapphire and diamond dress ornaments here of about 1750, for instance, conceived as sprays of cornflowers set among golden wheat, were just two of 20 that adorned the bodice of Elizabeth Petrovna, daughter of Peter the Great.

These ornaments were among the Russian Crown Jewels listed in an inventory of Russia's Treasures and Precious Stones compiled by the Commissariat of Finances in 1925, and subsequently sold to raise hard currency through private treaty sales and at Christie's in London in 1927. It was through such private sales with the Antiqvariat that Emanuel Snowman of Wartski was able to begin to buy Russians jewels, Easter eggs and sundry bijoux in 1925, and it has remained a speciality of the firm ever since. Now that the demand for Russian works of art is at a premium it is hard to remember that it was not ever thus. In 1927, the safe, courtly Louis Seize style of these cornflower ornaments, for instance, looked decidedly unfashionable in the face of emergent art deco.

But as this show reveals, there is more to Russian jewellery than Fabergé as well as safe, courtly taste. As well as tiaras from Fabergé's great rival Bolin, there is a monstrous dragon jewel by Lorié that is pure art nouveau, and from Denisov-Uralsky, an emerald and diamond-set geometric jewel set that prefigures Secessionist Vienna.

It is only now that the Iron Curtain has rusted that there is access to archivesin Russia and new scholarship has revealed the significance of many of these pieces. Research by the show's curator, Katherine Purcell, has shown that the fortunate owner of a pair of Fabergé cufflinks in the form of Easter eggs is in fact the possessor of the birthday gift of the Dowager Empress to her son, Nicholas II, in 1907.

One of the most delectable pieces in the show is an "ice" pendant, one of a series of jewels inspired by icicles and frost that were designed by Alma Pihl and confected out of carved rock crystal and crazed with diamond-set platinum. It is exhibited with its original drawings in the design book of the Fabergé workmaster Albert Holmstrom dated 1913 - like the Winter Egg from the same series that sold at Christie's in 2002 for a record $9.6m.

Most poignant is the perfect sea-blue aquamarine mounted in a brooch with a delicate tracery of interlocking diamonds. A tiny stock number scratched on the reverse was checked against the database compiled by Valentin Skurlov and Tatiana Fabergé and the jewel was discovered to have been a gift from the Tsarevitch to his fiancée Princess Alix of Hesse before their marriage in 1894. In the language of the lapidary, the gem stones represent eternal love. These jewels offer a window on to a lost world, and unlike all those hauntingly beautiful Romanov photographs, they will endure unfaded and un-diminished.