Some fiber and fabric types are the result of a process that takes a natural material and, through the powers of technology, transforms it into a wearable item. These hybrid fiber types can be difficult to understand when you're looking at labels in a store. What's natural, what's synthetic? Sometimes the best way to understand it is "a little bit of both".
Take rayon, for example. Aka "viscose", it was first patented in 1894 as an alternative to silk. It was an innovation spurred by a crisis in the European silk industry, which was threatened in the 1860s by a disease that killed silkworms. Seeking alternatives, the inventors went to work on a fiber that felt just as smooth and could serve as a good substitute, preferably without the risks and costs of silk.
They found their answer in cellulose; in other words, wood pulp. The wood is broken down by physical and chemical means, to generate a soup that can be turned into fiber and fabric; after all, what is paper made of? Unfortunately, the process of making rayon can require a huge amount of fossil fuels and toxic chemicals. It's come under fire in the sustainability movement as a fabric requiring too many resources in the wrong way.
But improvements have been made over the years to make the process more efficient and less polluting, and now there are varying degrees of "good" and "bad" rayon and viscose out there. Some companies like Lenzing use fast-growing woods like beech and eucalyptus, and manufacture in a closed-loop system that reduces or eliminates waste, emissions, and pollutants. But you need to look for special names on your clothing tags to get the good stuff-- looked for the trademarked names of Tencel, Modal, and MicroModal.
As is often the case, it's a combination of the original fiber, how it's grown, what it's processed with, and what chemicals it's treated with that makes it either a wonderful or a terrible choice for your skin and your values. Natural or synthetic, or semi-synthetic, the crucial difference is often the companies involved in the supply chain.