This is based on a true historical event in 1785. She was punished for artfully defrauding a master jeweler to get in possession of a priceless diamond necklace.
Since she chose to involve (without their knowing) the Queen and a cardinal in her scheme this event further added to the deterioration of the royal standing and was one of the sparks that ignited the French Revolution.
This movie choses to give some romantic background to the fraud scheme (which I doubt it really had).
https://en.wikipedia.org/wiki/Affair_of ... d_Necklace
https://www.4shared.com/s/fTLdpzdctjq