be set in stone

4. N-UNCOUNT : oft with brd-neg.

Stone is used in expressions such as set in stone and tablets of stone to suggest that an idea or rule is firm and fixed, and cannot be changed.

• Scientific opinions are not carved on tablets of stone; they change over the years.
  

... Collins cobuild

be carved/set in stone: if an arrangement, a plan, or a rule is set in stone, it is completely fixed so that it cannot be changed. 

• The rules aren't set in stone; they can be altered to suit changing circumstances. (usually negative) 
• These are just a few ideas to be discussed - nothing is carved in stone.
  

... Cambridge Int'l Dic of Idioms