Anne Somerset at Literary Review:
Packed with events so implausible a novelist would blush to include them in a work of fiction, the Diamond Necklace Affair had lasting consequences. In 1785 a fraud perpetrated for short-term gain by a shameless adventuress sowed 'the seed of the Revolution' that took place in France four years later, playing such a crucial part in Marie Antoinette's downfall that Napoleon later opined, 'The Queen's death must be dated from the diamond necklace trial.'
The adventuress in question, Jeanne de La Motte-Valois, boasted descent from a bastard son of a 16th-century French king. Having grown up in penury, she set about regaining her fortunes at the court of Louis XVI by duping Cardinal de Rohan in the most spectacular fashion imaginable. A scion of one of the grandest families in the land, Rohan had the prestigious position of Grand Almoner, but Marie Antoinette could not abide him. Rohan deluded himself that if he overcame her aversion, high ministerial office awaited him. Jeanne artfully exploited his gullibility by persuading him she held the key to the queen's favour.
Although the queen had walked on by when Jeanne tried to attract attention by staging a fainting fit, Jeanne convinced Rohan that Marie Antoinette had shown regal concern and that, having made her acquaintance in this way, she had progressed to becoming a royal confidante.