One of the most resistant biological material is

One of the most resistant biological materials is Sporopollenin.
Sporopollenin is a ubiquitous and extremely chemically inert biopolymer that constitutes the outer wall of all land-plant spores and pollen grains. It is a highly cross-linked polymer composed of carbon, hydrogen, and oxygen that is extraordinarily stable.
It is formed from the polymerization of carotenoids, is chemically very stable, and is resistant to almost all kinds of environmental damage.