Guppy, a gem dealer, of Ladbroke Grove, west London, and Benedict Marsh, a business associate, of Southwark, south London, both 28, were convicted at Snaresbrook Crown Court of conspiracy involving fraud, theft and false accounting.
They were remanded in custody and will be sentenced after they have stood trial on charges of VAT evasion. Judge Andrew Brooks warned them: 'I don't want you to leave the dock with any illusions. You are both going to prison for a very long time.'
The court heard that Guppy, a self-confessed gold smuggler, carried out a 'bold, well-researched, meticulously executed' fraud.
James Curtis, for the prosecution, described how, in March 1990, Guppy and Marsh paid a security expert, Peter Risdon, pounds 10,000 to pretend to be a gunman in a fake robbery of emeralds, sapphires and rubies insured for pounds 1.8m. In room 1208 of the Halloran House Hotel in New York, the pair drank champagne before Risdon tied them up and fired a bullet into a mattress.
Earlier the pair had called on Fifth Avenue gem dealers, including Tiffany's, deliberately asking far too much money for the precious stones. 'They wanted to be able to tell their insurers they had tried to sell them,' the court was told.
A New York policewoman described how she found Guppy and Marsh sweating and frightened. As Marsh claimed that two raiders had robbed and threatened to kill them, Guppy sobbed. The officer was fooled by the double act. 'It was a brilliant performance,' said Mr Curtis.
Days after returning to England the 'streetwise' pair secretly returned to America on Concorde to collect the gems, stashed in a safety deposit box, he said.
Using false invoices, provided by Ishan Dutta, a Bombay-based importer-exporter who was at Oxford University with them, the two men fooled Lloyd's into believing the gems had been bought from an Indian company. Dutta, 28, last year served six months of a jail sentence after admitting his involvement.
When they had received the Lloyd's insurance money, Guppy and Marsh performed the second part of their 'double sting'. They swapped stocks of gems between their Jermyn Street firm, Inca Gemstones, and a Geneva-based company in order to steal pounds 1.2m of the pay-out.
Guppy, who took a first in history at Oxford and was best man at Earl Spencer's society wedding to Victoria Lockwood in 1989, boasted of the 'perfect crime'. In court he was accused of a 'fondness for the unusual' and a tendency towards a 'James Bond, Walter Mitty' existence.
Mr Curtis said that police had been unable to recover the insurance cash or the money the pair were believed to have received for some of the 'stolen' jewels. Attempts by detectives to examine two private Swiss bank accounts have been blocked by legal proceedings launched by the two men in Geneva.
As the court rose, Marsh's girlfriend, Katrina, grabbed Mr Curtis's arm and insisted: 'You know he did not do this, he did not do this.' Mr Guppy's pregnant wife Patricia, mouthed the words: 'It's all right.' Guppy whispered: 'I love you.'