carved in stone

If a suggestion, plan, rule, etc. is carved in stone, it cannot be changed.

无法改变的;铁板钉钉的

These proposals are for discussion, they're not carved in stone.

这些提议是供讨论的，并非不可更改。

The deal isn't yet carved in stone, but we're confident it will go ahead as hoped.

这笔交易还不是板上钉钉的，但我们相信它会如预期进行下去。