Natural
noun, の adjective
Something of heaven's nature is indeed natural, with no human intervention involved.
Although this word is technically a noun, it's often used as an adjective. That's also what distinguishes it from 自然. So you'll often see 天然 used to say that something is "natural" or "of nature", while 自然 will often be used to talk about nature in general, like saying that nature is beautiful.
You know the reading for 天, but 然 uses a different reading from what you learned. So here's a mnemonic to help you:
One thing that definitely isn't natural are nendoroids (ねん). Remember, the little plastic figurines? Have you ever seen an anime figurine growing naturally in a field before? Yup, me neither. Not very natural.
この池はたくさんの人が泳いでて、まるで天然のプールだ。
This pond has a lot of people swimming in it, it's like a natural pool.
コンビニでペットボトルの天然水を買ってきてくれる？
Would you mind buying a bottle of natural water from the convenience store?
天然のウナギやブリは高いです。
Wild eel and yellow tail are quite expensive.