London CNN  — 

A 60-year-old French woman who posed as a diamond expert entered a luxury London jewelers and stole seven precious gems in an elaborate daylight heist has been jailed for five and a half years, police have said.

Lulu Lakatos, who was part of an international organized crime gang, visited the Boodles store in central London in March 2016 and played the role of a gemologist sent to inspect seven diamonds worth £4.2 million ($5.8 million) on behalf of a wealthy Russian investor.

But Lakatos, who pretended she couldn’t speak English in order to buy herself time in the store, placed a bag filled with the diamonds into her handbag and replaced it with another bag filled with pebbles.

She was found guilty of conspiracy to steal and sentenced at Southwark Crown Court in London on Wednesday.

The complex heist had been planned well in advance, London’s Metropolitan Police said in a press release. Lakatos entered the UK the day before the raid and stayed in a hotel, before scoping out the store with two men. The next day she returned dressed as “Anna,” the diamond expert she would be impersonating.

Police said that she was taken to a secure part of the store to inspect the gems, and that she alarmed staff when she placed them in her handbag.

But after an exchange in which Lakatos acted like she was struggling to communicate in English, she allayed concerns by apparently removing the diamonds from her handbag. Staff were unaware until after she had left the store that Lakatos had, in fact, taken out a different bag filled with worthless pebbles.

Lulu Lakatos impersonated a diamond expert during the raid.

“This was an audacious theft, carried out in plain view of experienced and professional staff at a renowned jewellers,” acting detective sergeant William Man of the Metropolitan Police’s Flying Squad said in a statement. “The meticulous planning and execution of this theft reveals to me that those involved were highly skilled criminals.”

“While she played a key role in this theft, it is clear she did not work alone and enquiries remain ongoing to identify all those involved,” he added.

The two men involved in the theft were also convicted of conspiracy to steal, while an investigation is ongoing to find two women seen with Lakatos during the operation.

Boodles is a luxury jewelers in London’s upmarket Mayfair region.

The city’s jewelery quarter is a frequent target of planned heists – most notably a 2015 raid in which a group of men stole more than $20 million worth valuables from a vault.