A woman stole diamonds worth 4.2 million pounds ($5.7 million) from a luxury London jeweler and swapped them for garden pebbles using "sleight of hand," a court has heard.

Lulu Lakatos, 60, pretended to be "Anna', the gem expert, and traveled to Boodles in London's Mayfair neighborhood in March 2016 to appraise seven diamonds for wealthy Russian investors, The Guardian said.

The diamonds were to be placed in a locked bag and held in the jeweler's vault pending a transfer of funds.

The gems included a 20-carat heart-shaped diamond valued at more than 2.2 million pounds.

The next day, Boodles' own gemologist became suspicious and when she opened the bag, she found seven small pebbles.

"The diamonds had been stolen by the defendant by sleight of hand," The Guardian quoted the prosecutor, Philip Stott, as saying.

Lakatos, from the Saint-Brieuc region of north-western France, denies any wrongdoing. She has been charged with conspiracy to steal.

Boodles is one of Britain's premier luxury jewelry brands. Nicholas Wainwright, its chairperson, said he had been approached by an Israeli buyer who wanted to invest in high-value diamonds in the weeks before the theft.

According to ABC News, Wainwright met with Lakatos in his shop along with his own diamond expert Emma Barton.

As soon as Wainwright left the room to take a call from the alleged buyer, Lakatos placed the padlocked purse containing the diamonds in her own handbag, Barton told the jury.

"The locked purse-like bag containing the diamonds was then placed inside the shop's safe, and the defendant walked out of the shop," Stott told the jury.

"It seems what happened was it was swapped for an identical locked bag and that duplicate bag was placed back on the table," Stott said.

Stott said Lakatos then worked with accomplices to make their getaway to France in a rented car. She then used her own passport to leave London on the Eurostar, the court was told.

Authorities arrested Lakatos in France on a European arrest warrant in September before being extradited to the UK.

The trial continues.