I have always seen the expression 'carved in stone' used in the negative in sentences like: 'It is not carved in stone that..., Could this expression be used in the positive or, alternatively what other expression could I use to say that something is 'unchangeable' ?
A review of carved in stone in an ngram reveals mostly either literal usage (describing actual stonework) or negative usage.
There are occasional positive uses such as this description in Shark, Lauren St. John's biography of Greg Norman
He could not have been more confident of the outcome if it had been carved in stone beside the tee box.
A quote from Primo Levi about Holocaust survivors uses the phrase to indicate an indelible memory
The survivors are divided into two well-defined groups: those who repress their past en bloc, and those whose memory of the offence persists, as though carved in stone.
The phrase forms the title of Manny Drukier's book, Carved in Stone: Holocaust Years, a Boy's Tale
In Modern American Usage, the authors acknowledge the common negative connotation, while pointing out the availablity of the affirmative meaning
Being democratic, we wish to accommodate others, hence the reassuring statement, always negative, that a particular rule, decision, or practice is not carved in stone; i.e., it can be changed. In the positive, the image comes quickly to mind: laboriously carving in the nearly indestructible stone.
You can say that something is set in stone to indicate that a particular arrangement, document etc. cannot be changed. (Its opposite is, logically enough, not set in stone.)
firmly established and very difficult to change | set in concrete