Of the established eggshell groups (membrane-like, pliable, and rigid), the rigid eggshell group has the best chance of fossilizing. Fossils of this group, with modern-type structure, extend back into the Eocene (crocodiles, gecko) and even into the Cretaceous (birds, turtles).
Structural types which differ from modern types are found as late as the Eocene, and in the Cretaceous they are numerous. These Cretaceous eggshells have, for the most part, been assigned to dinosaurs often without consideration of other egg-laying animals of that time. Only a few eggs and eggshells have been reported from the Jurassic and older periods.
Polarizing and scanning electron microscopy complement each other. For example, the polarizing light microscope shows the extinction pattern and the larger units of the shell structure whereas the scanning electron microscope allows a detailed study of the microstructure which may enable us ultimately to identify specimens to lower taxonomic groups.
Hirsch, Karl F. and Packard, Mary J.
"Review of Fossil Eggs and Their Shell Structure,"
Scanning Microscopy: Vol. 1
, Article 35.
Available at: https://digitalcommons.usu.edu/microscopy/vol1/iss1/35