The structural material forming the exine or outer layer of pollen grains and microspores.

Sporopollenin is a highly stable material, conserved in sedimentary deposits over very long geological time, and has allowed detection of pollen from 500 Myr ago and microspores from 3.5 Gyr ago. It is the most resistant of all natural organic compounds

In its free form it is yellow-brown, in particles that may be 30 µm across. Its chemical composition is uncertain but it some believe it to be an oxidative polymer of carotenoids and carotenoid esters. Its composition was first probed in 1931.

Further reading.
http://www.polysciences.de/data_sheets/281.html
http://www-class.unl.edu/geol846/intro.html