The car, a Jaguar being driven by Christian Klien, had the diamond embedded in its nosecone as part of a promotion for the film, which features a jewellery heist, and the Israeli diamond merchant Steinmetz.
However, when Klien crashed his car into a barrier early on in the race, the diamond disappeared and has still not been recovered.
The Monaco promotion is part of the push for 'Ocean's Twelve', which this time sees Clooney, Pitt and company attempt three European heists in Rome, London and Berlin while 'Ocean's Eleven' baddie, casino owner Andy Garcia, is hot on their heels after they ripped off his casino in Las Vegas in the first movie.
But one publicity expert has suggested that the gem was unlikely to be real.
Robert Phillips, founding partner of consumer PR agency Jackie Cooper PR, said: "If it was my client, I wouldn't have put a £200,000 diamond in the front of a Formula 1 car."
He went on to say that it was a great publicity stunt for Steinmetz and for Jaguar and for the sequel to hit to 2001's orginal 'Ocean's Eleven'.
"It was also great publicity for them to have pictures of Brad Pitt diving off his yacht in Monaco all over the papers on Saturday," he said.
Nav Sidhu, spokesman for the Jaguar team, has admitted that the idea to embed the diamonds on the front of the car was his idea, and said: "I don't expect we are going to get it back."
Even if the diamond is real, Steinmetz is not about to be ruined by the loss, with reports that it is one of the biggest players on the diamond market and that it is negotiating a deal to export more than £50m of diamonds a year from Russia.
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