Photoresistor to detect materials

Hi there, is anyone familiar with using Photo-resistors to detect material of an object, ex- Al, plastic, or paper. I'm under the impression that we'll have to do it together with RC time method, but that's all i got.
Or is there a better solution for detecting plastic, aluminum and paper? using one method to detect them all? I think we can use capacitive sensing as well, no?

Anybody with any ideas would be greatly appreciated! 😀