Sitting next to a high-end London jeweler, a French woman wearing a black hat and red scarf examined a bag holding several million dollars’ worth of diamonds placed before her.

She then put the bag in her purse, surveillance footage from March 2016 shows.

Alarmed, the gemologist explained she couldn’t leave with the diamonds until the store confirmed payment. The French woman apologized, stalling with her hand in her purse as she blamed the misstep on her poor English. Moments later she retrieved what appeared to be the bag of precious gems.

It was not until the next day when the gemologist realized she’d been duped, police said. The French woman had left the store with diamonds worth today’s equivalent of $5.8 million in her purse — leaving the gemologist instead with a bag of seven worthless pebbles.

After years of investigating the heist, police identified the thief as Lulu Lakatos, a now 60-year-old member of an international organized gang. She is from the Saint-Brieuc region of northwest France.

Law enforcement apprehended Lakatos in France and extradited her to London on Dec. 3. On Wednesday, she was found guilty of conspiracy to steal and sentenced to 5½ years in prison, according to a London Metropolitan Police news release.

“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, which investigates organized crime, said in a statement. “The meticulous planning and execution of this theft reveals to me that those involved were highly skilled criminals.”

Although Lakatos “played a key role in this theft,” Man said, she had help from other members. Metropolitan Police arrested two additional men involved in the scheme. Both were convicted of conspiracy to steal.

The meticulously planned raid could have been plagiarized from a heist novel. For weeks leading up to the theft on March 10, 2016, gang members posing as associates of a wealthy Russian investor met several times with Boodles, a high-end London jeweler.

The supposed deal culminated with Lakatos posing in a meeting as the investor’s personal gemologist, who would be sent to London to vet the diamonds.

Lakatos arrived in London one day before the heist, police said, and checked into a hotel. At about 8:15 p.m., Lakatos met up with two male gang members at a nearby cafe. They then drove to Boodles where they “scoped out” the store, police said.

In a full-length trench coat, black hat and large red-and-purple scarf wrapped around her neck, Lakatos arrived at Boodles the following morning for the prearranged meeting. She introduced herself as Anna.

She wasn’t alone, police said. Surveillance footage showed four gang members standing outside the store — the two men Lakatos was with the night before and two women police have not yet identified.

Lakatos informed the employees after arriving at her appointment that she did not speak English very well — a tactic she used to help pull off the theft, police said.

A Boodles gemologist took Lakatos to a secure area, where she produced the seven diamonds for examination. Each diamond was individually placed in a locked bag that would remain at the store until it received full payment from the Russian investor.

After reviewing the diamonds, Lakatos closed the bag and placed it in her purse.

“The Boodles gemologist immediately challenged her, but Lakatos used the apparent language barrier to cause a delay, before appearing to produce the same locked bag containing the diamonds from her handbag,” police said in a news release.

Those few seconds were key for Lakatos, investigators learned. She used her supposed language barrier to stall and covertly swap the bag of diamonds with an identical one filled with seven pebbles of the exact weight as the gems.

Still a bit wary, the gemologist searched Lakatos’s bag but found nothing suspicious.

Lakatos soon walked out of the store with the diamonds secured in her purse. Surveillance footage from down the street showed her walking alongside the two women who had been waiting outside Boodles with the two men trailing behind them. Lakatos can be seen handing the bag to one of the women before going her separate way.

Lakatos then hopped in a taxi, which took her to a pub near Victoria Station, police said. She changed in the bathroom and made her way to a train, which took her out of the country.

“She had been in and out of the country, having committed the theft, within 24 hours and within just three hours of committing the offense,” police said.

The following day, the gemologist at Boodles remained uneasy about her interaction with Anna. To be sure, she X-rayed the locked bag. Something wasn’t right, she thought, according to police.

She opened the bag and found the pebbles in place of the precious diamonds.

The Metropolitan Police’s Flying Squad soon got on the case, and for years pieced together hours of security footage, following the gang members’ every move.

Despite Lakatos’s conviction, police are still investigating whether others were involved.

“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,” Man, the acting detective sergeant, said.