The plan, involving Anne Hathaway's character Daphne Kruger, involved switching the original 6 pound diamond necklace with a fake, 3D printed or digitally machined from zirconium crystal- which many may link to the popular diamond imitation; cubic zirconia.
Now what most people don't know, but have probably speculated, is that even the real necklace depicted in the movie was actually a fake. Cartier took about 56 days to create the stunning piece of faux jewelry using several giant pieces of cubic zirconia and white gold.
The illustrious "Jeanne Toussaint" necklace was named after the brand's creative director (1930's), and was directly inspired by a former piece of jewelry created for the Maharaja of Nawanagar by Jacques Cartier, which shows that the movie's maestros wanted to incorporate a deeply historical and true-to-life feel for the masterpiece's aesthetics.
The movie budget would have skyrocketed by over 30 million dollars (more) had actual diamonds been used in-place of zirconia. Although despite the main piece being fake, several authentic diamond pieces actually make cameos throughout the film, especially during the scenes at the Metropolitan Museum of Art. Cartier had a lot of participation in the development of the movie, loaning several of their fine works on-site to create more realistic exhibition display scenes.
