Something present in nature or produced by nature is natural.
Earthquake, atoms, humans, and death are natural. All clothing is artificial, that is, not natural. Bearded men are natural. Face masks on a nudist beach are unnatural. H2O is natural. The same is CO2. The temperature changes are natural. Occasional dry summers are natural.
Some plants are lethal even though they are natural. But if you die from eating a lethal but natural plant, you will not be said to have died of natural causes. If you die from lung cancer caused by smoking tobacco, a natural plant, you will be said to have died of natural causes.
Eating meat might be a natural act, but eating cooked meat is unnatural. Civilization is unnatural. So are auto-mobiles. Urinating whenever one has the urge is natural, but uncivilized. To have a broken arm set by a physician is unnatural. Getting a medical degree is unnatural. Foraging and experimenting by trial and error would be natural, if often lethal.
Is your smartphone unnatural, even if it talks? Talk to Bixby or Siri! Turning the other cheek when struck is considered Christian, but it is unnatural. Is Islamic call to prayer natural, or just supernatural? Anything supernatural is unnatural. But it is usually considered good by those who believe in the supernatural.
So why do so many make a big fuss about using only what is natural? Such an obsession seems unhealthy. But it helps one avoid having to ask difficult questions about whether something really is good, safe or healthy. All you need to know is that something is “natural”. Not thinking comes naturally to some people. Belonging to some majority is "natural", "safe" and "good", but often not that good. Consensus may very well delay the emergence of new knowledge.
Everything that is made is comprised of nothing but natural atoms.