Diamond geezer's advice on ice

There's something exquisitely illicit about even the most legitimate diamond dealer - the money, the paraphernalia, the clientele. It all conjures up images of a glamorous, if sometimes dangerous, life. From Diamonds Are Forever to Marathon Man, film producers have long realised the gangster-chic appeal of these sparkling gems.

However, diamond dealer Joel Grunberger, 38, had never seen a movie where his world was properly portrayed. So when he found out that Guy Ritchie, director of Lock, Stock And Two Smoking Barrels, was basing his next film on a diamond robbery, he knew he was the man to give "advice on ice".

After hearing from a friend in the film business that Mr Ritchie's forthcoming film, Snatch, would centre on the Hatton Garden diamond trade, Mr Grunberger approached Ska Productions nearly three years ago. "There's a lot of Yiddish bandied about in this business, not only by Jews but by everyone - Indians, Anglos and Dutch - and Guy was very keen on making the characters and the dialogue as realistic as possible," Mr Grunberger said.

"At first Guy was putting phrases in the dealers' mouths that they just wouldn't use. Doug the Head, played by the EastEnders actor Mike Reid, was using totally inappropriate phrases. A dealer wouldn't describe a diamond as being 'a lovely colour and very pure'. He would say, 'That's a D colour, two-carater, round VVS'."

A "D colour" is the pinnacle of a diminishing scale of quality - there is no A, B or C. With D, E and F the best, G, H and I are nearly colourless and of lesser value, while the quality of those from J through K and so on declines as they "draw colour" of various shades of yellow.

The carat is the weight, the "round" refers to a brilliant shape and the VVS, denoting Very, Very Small, refers to the microscopic size of the stone's internal imperfections. The best are "internally flawless" and the worst are "pique'd", from the French for stung.

Mr Grunberger added: "I had to tell Guy where it would be appropriate to throw in bits of Yiddish. Everyone knows phrases like putz and schmuck, and even schlepping (owing money) has entered everyday-speak, but there are others used specifically by diamond merchants."

After haggling over the price of a diamond and finally coming to an agreement, he said that deals are sealed with the Hebrew phrase "mazel u bracha", which means good luck and good blessing.

This is as close to a contract as there is, to the extent that if after this point either party backed down, the other could take him to the Diamond Bourse, the trade's regulating body in London.

Mr Grunberger continued: "At times I had to curb Guy's street mentality. I had to let him know that a religious Jew, even a quasi-religious Jew who eats pork, like Doug, just wouldn't tell somebody to 'f*** off'."

The film's set designer also used Mr Grunberger's office as a blueprint for Doug's, and equipment was borrowed to add extra authenticity, including special lamps which give out a white "northern" light, and specialist scales and sieves.

While advising on the business's vernacular, Mr Grunberger also explained about the most up-to-date diamond replica, moissanite, which has replaced cubic zirconium as the stone of choice for dodgy dealers palming off counterfeit goods. "Honest dealers in Hatton Garden work cheek-by-jowl with villains. I don't mean they sell to villains but within the business there are unscrupulous people. Everyone knows the Brinks Matt haul came to Hatton Garden and a street very close to here, Greville Street, has a very different vibe to Hatton Garden. There's a tangible edge and the police know that."