They're polystyrene and come on sprues of 4, and you get 100 for $22. The edges are beveled so they fit tightly against a 25mm base, leaving no gap on the surface of the base.
|Left to Right: Conversion Ring, Conversion Ring on 25mm Base, 32mm Base|
|Converted 25mm base on left, 32mm base on right|
In the end, I'm really happy with the Eccentric Miniature conversion rings. I'll admit that they're not the best design due to the issues mentioned, but they do a good job for an excellent price. If you want to rebase a large Marine army, they provide an economical way to do it.