She said that the designer was "rare and special" during a live online interview to promote the fashion house's new fragrance Florabotanica, which she is the face of.
Stewart explained: "I had to have a relationship with fashion from such a young age - you walk down the red carpet, and it matters. It's strange.
"At first I just wanted to get it right, I wanted it to be smooth sailing and just to be able to do my job. Then I did a photo shoot with Nicolas and it was the first time that taking photos in high fashion felt like telling a story, that it was part of a concept - it wasn't just about something on the surface, it really evoked a feeling."
The Twilight actress also spoke about her personal style during the interview, saying: "I'm from LA and unless the occasion calls for it, I'm a really simplistic, comfortable, straightforward dresser - I don't really think about it.
"I go anywhere outside of LA and I'm like, 'Oh God. I forgot to put socks on. I probably look silly, it's winter time.'"