Even broken into a thousand pieces, the gem would have been identified

The largest flawless pear-shaped diamond in the world, surrounded by 12 of the finest gems of their kind, was a temptation too great to ignore for the robbers who attempted to raid the Dome yesterday.

The largest flawless pear-shaped diamond in the world, surrounded by 12 of the finest gems of their kind, was a temptation too great to ignore for the robbers who attempted to raid the Dome yesterday.

The £350m pound jewels, which formed the centre-piece display of the "diamond experience" in the Money Zone, belong to De Beers, the world's largest diamond-mining company.

Pride of place in the exhibition is the magnificent Millennium Star, a two-inch long, 203-carat stone which took three years to cut and polish after it was found in the Democratic Republic of Congo (then known as Zaire) in the early 1990s and purchased uncut by De Beers.

De Beers chairman Nicky Oppenheimer said of the diamond: "My father, who has handled many of the world's most important diamonds, has declared that it is one of the most beautiful diamonds he has ever seen."

Also in an Dome display were 11 other "blue" diamonds in sizes ranging from 5.16 to 27 carats. These diamonds were mined at De Beers flagship mine in South Africa which yields one significant blue stone a year. The largest is the 27-carat Heart of Eternity, which is of a particularly intense blue.

When it was announced in autumn 1999 that the diamonds were going on display in the Dome, Mr Oppenheimer said: "Security is always a concern but we like to think we have a pretty good handle on that."

Yesterday those security plans were put to the test when robbers tried to smash through the reinforced security glass protecting the gems. However, even if they had succeeded in breaching the security, De Beers admitted yesterday that they had taken the extra precaution of substituting worthless replicas for the originals after the police informed De Beers of the expected robbery attempt.

Had the robbers been successful in pulling off the world's biggest-ever jewell heist, their problems would have been far from over. Selling the diamonds, especially ones so easily identifiable, would have presented immense problems.

Each diamond would have been instantly recognisable by dealers. They would have had to be cut down to smaller stones to be put on the market, and even then diamonds from different "pipes" or seams are detectable to experts.

Any London-based robbery gang capable of organising such a daring raid would have likely already planned the "fencing" or sale of the jewels. It is also probablethat they had connections in London's diamond district at, Hatton Gardens. It is speculated that many top criminals have connections with "The Garden". Alongside the reputable jewellery trade are less-scrupulous dealers trading in gold and jewels.

The robbers would have perhaps hidden the diamonds until the hue and cry of the theft died down. Then their Hatton Garden "fences" would have employed dishonest cutters and polishers to disguise the gems. Reduced to small diamonds, the haul would have been slowly put on the market through the international diamond markets in Antwerp and Tel Aviv.

Although much of the value of a diamond like the Millennium star is in its sheer size, and cutting down would have substantially reduced the value of the haul - even part of £350m - is enough to put a sparkle inthe eye of the most hardened criminals.