Calcium Oxalate stone - can't remove

The vet told me that my cat has a calcium oxalate stone but that it's
located in a place where they cannot perform surgery. They're just
gonna have to wait and see what happens with it and keep an eye on it.
I can't remember the location of the stone but I think it was above or
around the colon. Does anyone happen to know what/where it might be? I
was going to do some research on it but the vet isn't open on Sunday so
I can't ask. I think the location started with the letter U, but I'm
not sure. I don't think it was in the urethra or the bladder, though.

Any ideas?