MAC stuff is usually mica/synthetic, but I know with other products (those that aren't veggie-friendly) they can even be fish scales... they can be synthetic... but to make you less afraid, I've had gold leaf and silver leaf applied on me in sheets and it's fine. The cosmetic grade stuff is similar to the edible grade. As long as it's a major brand I wouldn't be hugely cncerned because it's hella regulated. Lead is the big baddie, btw...