This fallacious reasoning claims something is better just because it is more natural. That can indeed be the case, but it depends on the claim. Poppy plants are natural. Therefore opium is good for you, right? Wrong. Wine is made of grapes, but too much of a good thing is harmful. And you wouldn’t want poisonous belladonna leaves mixed in your salad. Just because something is natural, doesn’t make it better than the synthetic alternative. It could be better, for sure. Be claiming it is per definition better is fallacious.
Argumentum ad Naturam