I have one, and it's aluminum. Some of the off feel may come from the coating on the image side, which is similar to limestone, but isn't that.
There may have been plastic plates with the same coating, the place I worked only used the aluminum stuff, and I haven't really looked into when they started using other materials. _ My google skills must be improving, Plastic plates for offset lithography didn't come out till 1967.
http://www.radtechmembers.org/archiv...papers/121.pdf
It's possible to use almost anything, the litho we did in art class was done with a plate that was paper.