That is a standard GM dome light lens, and the lens is made of plastic. Yours might have become hardened as a result of age, but if you use a small flat screwdriver, you should be able to pry the lens from the metal bezel. You can purchase a new lens on ebay, however, you will have a more difficult time finding the star-burst design lens, than the replacement that GM started using in the later 1960's & 1970's. The star-burst design goes back the the late 1950's & early 1960's. The bezel also went from a metal product to a brushed chrome plastic unit about the same time. If I remember correctly yours uses a special bayonet type light bulb, #89, with two electrical contacts on the bottom.