One of these?



Slightly amusing, my hammer and anvil stones work on the same principle. I have a biggish stationary rock that has a point on it. I hold the plate in my left hand and a hand-sized hammer stone in my right, which also comes to a decent point. Guide the plate between these two points, and it is pulverized very precisely (usually) at the point/s of impact. Just work in from the edge until you're happy. Kind of like using a nibbling tool to shape metal.

Do this somewhere sensible, because you end up with a lot of splintery debris!