The actress has starred in high-profile campaigns for the fashion house and when she learned she'd have to be bathed in baubles for the adaptation of Leo Tolstoy's classic novel, she didn't hesitate to call on a couple of high-ranking Chanel pals.
She says, ""We phoned up Chanel and asked, 'Can we use your jewellery please?' and they said, 'Yeah,' so that was great.
""All of the jewels that I'm wearing in the film are all real, so it was a lot of diamonds and pearls and a couple of rubies in there as well. I didn't get to take any of them home with me, unfortunately; they had to all go back to Chanel but it was very nice getting dressed in the morning.
""Do you know it's terrifying how quickly you take it as normal. You get a two million pound necklace, or whatever it was, and before long you're just kind of playing with it, like, 'What am I doing?'""
And there was one Chanel brooch that Knightley wore, as the title character, which even wowed her director, Joe Knight.
He tells WENN, ""The jewellery was extraordinary. There's one brooch that she wears in a scene where she is pregnant which was worth, like, $2.5 million - and she just arrived on set wearing this thing with this pregnant belly in this kind of maternity dress.
""I said, 'Surely that's too much Keira for the scene? She's pregnant at home and she's wearing this outrageous brooch.' She said, 'This is exactly why she is wearing the brooch - because she's pregnant.'""