I can only guess because they're no longer on the website. For the price and feel, I believe they must be mostly natural. Looking back at the website, it looks like the good sets are a combination.
answered 1 year, 8 months ago
According to Sephora:
Powder brush, Blush brush, Fan brush, Shadow brush, Crease brush, Smudge brush and the Brow brush are natural haired.
Foundation brush, Concealer brush, Angle liner brush and the Lip brush are synthetic bristled.