be written in stone

To be permanently fixed or firmly established; to be incapable of being changed, ignored, or avoided. Often used in negative constructions. The deal isn't yet written in stone, but we're confident it will go ahead as hoped. Can you tell me if that rule is written in stone? We would be a lot more productive if we didn't have to adhere to it every time. A: "Are these blueprints written in stone?" B: "No, they're just to give you a feel for the design."