Rubber can be either natural (from trees) or synthetic. Most yoga mats with claims of having a natural product use natural rubbers, which are then processed with chemicals to make them into a durable product. With this in mind, you should be skeptical to claims of 100% natural rubber products.
The environmental issue regarding natural rubber are far from being resolved. Increased demand for rubber means that more tropical rain forest will be cut to plant rubber trees. We understand that tropical forest is cut for essential food crops for the local population, but we do not condone the cutting of forest for industrial crops like rubber trees, which destroy tropical biospheres and the living environment for many animal and plant species.
It is a rather sad sight, seeing the diverse vegetation in areas like Kerala/South India, Malaysia and Indonesia disappear to make room for rubber mono culture. Very few plants are able to survive under the shade of a monoculture of rubber trees. As an example, South India is already experiencing a severe shortage of Ayurvedic herbs because of this.
After the rubber is harvested by hand it must first undergo chemical processing to maintain it and then further processing and additives to make it into a durable product. Natural rubber products will only withstand perhaps 2 years of use or less before actually breaking down and coming apart. Rubber breaks down under sunlight and oxygen exposure, and with high heat exposure will break down even faster.
For the user, a natural rubber mat will smell (something like tires) and usually contains latex, which is often what natural rubber is, a common allergen. One can expect to replace their mat after breathing its fumes in roughly 2 years. For us, rubber is not an option.