Changing Vanilla Items

So I want to edit existing Vanilla objects, but I'm not sure how. I know how to replace the texture that's on it, but is there a way to change a blocks properties? In my instance I want the hardness of the vanilla clay block to be higher so it takes longer to break and to only drop clay when mined with like a stone pick axe. I understand I can just make a block using these circumstances but is there anything I can do about the existing Vanilla clay block?