I've had ones made of something different to plastic and Bakelite. It is almost a ceramic, because it chipped when I dropped one, but I was able to use a sanding drum to remove the damage. I honestly don't know what the material is. The picture is of one that I had to sand the lip down because of some damage.
I think a lot of them may be cracked because they are filled with plaster, and the sealing has failed, allowing the plaster to get wet and swell.