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. She showed Rohan affectionate letters the queen had purportedly written to ‘my cousin, the Comtesse de Valois’, and then told Rohan that Marie Antoinette would be receptive to his overtures. Rohan wrote flowery letters that Jeanne undertook to convey to the queen and received, in return, gracious handwritten notes.

When Jeanne could no longer stave off Rohan’s demands to meet with the queen, she organised a midnight assignation in the gardens of Versailles. Having dressed a prostitute resembling Marie Antoinette in the sort of simple muslin gown the queen delighted in wearing, Jeanne arranged for Rohan to have a brief nocturnal encounter with this woman in the suggestively named Grove of Venus. Clutching the rose the shadowy figure had conferred on him, Rohan was ecstatic that the queen seemed so well disposed towards him. Thrilled that Marie Antoinette’s attitude had softened, Rohan eagerly pressed money on Jeanne when she told him that the queen was temporarily short of funds and would be grateful if he were to loan her cash for charitable donations. In reality, Jeanne pocketed these sums herself.

Rohan was not alone in believing that Jeanne was on intimate terms with the queen. In 1785 Charles Auguste Boehmer, the court jeweller, approached Jeanne in the hope that she could persuade Marie Antoinette to buy a necklace containing 647 diamonds that had initially been designed for the late king’s mistress Madame du Barry but which the jeweller was now finding hard to shift. So heavy and uncomfortable that it was termed a ‘necklace of slavery’, this elaborate and vulgar piece had already been rejected by Marie Antoinette. The jeweller was relying on her new female favourite to change her mind.

Scenting an opportunity to pull off an audacious coup, Jeanne told Rohan that Marie Antoinette wished him to oversee the purchase of the necklace and that the queen would honour any deal he reached with its owners. The cardinal negotiated a price of 1.6 million livres, with the first instalment payable six months after delivery. As soon as the jeweller had handed over the necklace, Rohan surrendered it to a man he believed to be a servant of Marie Antoinette but who was really one of Jeanne’s accomplices. The necklace was broken up and sold in London, and for a few months Jeanne and her husband lived luxuriously on the proceeds.

As time passed without payment from the queen, Rohan and the jeweller grew uneasy. Granted an audience with Marie Antoinette in August 1785, Boehmer insisted that she ‘condescend to admit’ to possessing the necklace. On learning that Rohan had claimed she had authorised him to act as her agent, the queen grew incandescent with fury. Rohan was summoned and told that it was incomprehensible that he ‘could have imagined for an instant’ that she would employ him on an errand of this kind, when ‘my opinion of you has been established for a long enough time’. For his part, Louis XVI assumed that Rohan had been motivated by simple criminality and that he had invoked the queen’s name to obtain a necklace he planned to sell for his own enrichment.

An earlier case in which a female courtier had fraudulently obtained loans using the queen’s forged signature had been hushed up, as a royal adviser realised that prosecution might have unwelcome repercussions. On this occasion the king and queen were advised by an enemy of Rohan that nothing less than exemplary punishment was appropriate. Although Rohan would have been willing to borrow money to settle the jeweller’s bill, the matter was referred to the law courts. Too late, the queen began to fear that criminal proceedings ‘could have other consequences’, while Talleyrand predicted that the affair ‘may well rock the throne of France’.

Months later Rohan was acquitted by a majority verdict, ‘a terrible insult’ to Marie Antoinette that left her ‘bathed with tears of grief and despair’. Even at court, many people suspected that she had employed Rohan to secure her the necklace and then disowned him when it suited her to do so. In the public’s eyes Marie Antoinette was rapacious and corrupt, and her reputation never recovered.

Not even the fact that Jeanne was convicted, and sentenced to be whipped and branded on both shoulders, could rehabilitate the queen. Jeanne had tried to deflect blame onto Count Cagliostro, a Sicilian alchemist and quack healer whose mystic pronouncements had fascinated Rohan; but Cagliostro successfully rebutted Jeanne’s claim that his illiterate wife had forged the queen’s writing. The perception nevertheless remained that Jeanne was an innocent who had been manipulated by the queen. After she had served a year in jail, sympathisers facilitated her escape. Having decamped to England, she produced a justificatory memoir hinting that Marie Antoinette was sexually involved with Rohan and herself. One scholar described it as a ‘foundational text’ of the French Revolution.

The scandal pursued Marie Antoinette to the very end. When the royal family were brought back to Paris in 1791 after their disastrous attempt to flee revolutionary France, a print was published depicting the queen defecating diamonds. At her trial in 1793, the public prosecutor confronted her about her dealings with Jeanne, whom he referred to as ‘your victim in the famous affair of the necklace’.

Jonathan Beckman skilfully unfolds the intricacies and absurdities of this extraordinary episode. Since accounts written by most of the protagonists are full of falsehoods, and so much of their behaviour defied logic, teasing out the truth is difficult, but Beckman provides us with an engaging and finely researched study of an affair that, despite having the plot of a frothy operetta, was of genuine historical significance.