There are quite a few ways to do that, the simplest is probably to use Split:
string inp = "STR_LOCAL_ABS_CERAMIUM_A21";
string[] parts = inp.Split('_');
parts[3]
will have "CERAMIUM" - if the "bit" you want is always in the same potion, that will do it.
If it's always in the same position relative to the end, then use
parts[parts.Length - 2]
instead.