(IDEX Online) - A bogus gemologist swapped diamonds valued at $7.8m for worthless pebbles in an audacious heist, a court in London has heard.Lulu Lakatos, 60, is alleged to have examined the gems at luxury Mayfair jeweler Boodles (pictured) on behalf of a group posing as wealthy Russian buyers.She then placed the locked purse containing seven diamonds into her handbag, Southwark Crown Court was told.When staff protested she immediately withdrew what appeared to be the same purse, it is alleged, but had in fact swapped it for a replica, containing pebbles.Staff placed the replica purse in the vault and it was only when they checked the following day that they realized it contained nothing but garden pebbles.Among the diamonds stolen was a 20-carat heart-shaped diamond worth $3m million and a 3-ct pear-shaped fancy pink diamond valued at $1.5m.Lakatos, born in Romania, was extradited from France, where she lives. She denies conspiracy to steal on or before March 10 2016. The trial continues.Two men have already pleaded guilty to conspiracy to steal.