be cast in stone直接翻译是“刻在石头上”，可以翻译成“板上钉钉、无法改变”。

【例句】
These suggestions are for discussion. They are not cast in stone.
这些建议是用来讨论的，不是板上钉钉的。

These requirements must be met, but as for the way you make it is not carved in stone.
这些要求必须满足，至于你怎么做不是硬性要求。