Help with removing trim plate

[noshower]

My shower valve stem broke off and I cannot for the life of me, remove the trim plate to get to the valve and replace it.
The shower does not have any model name on it but when I took the part to HomeDepot, they said it could be price pfister. I do not want to damage/force the slate/marble it is stuck on.

I tried using a putty knife and a blade to remove the trim plate ..both unsuccessfully
I am attaching the pictures here.

Any help in how to remove this would really be helpful.

 

[havasu]

I'd recommend using a utility knife to break the bond from the putty or whatever glue the previous owner used to secure the plate to the wall.

 

[phishfood]

That does look like a Price Pfister. Not 100% on that, but I think it is.

If so, the plate is held on by the center trim "barrel". That will unthread, grab it with a pair of pliers to loosen it.