Etched In Stone

Is the correct saying

"It is not etched in stone" or is it

"It is not etched in granite"

which is the correct way to say that cliche
stone or granite which way have you heard it most referred to.
Thank you.

I vote for "etched in stone." And "Slang: the Authoritative Topic-by-Topic Dictionary of American Lingoes from All Walks of Life" by Paul Dickson (Pocket Books, New York, 1990, 1998) agrees with me. In the "Bureacratese" chapter, it has "etched in stone. Cannot be changed, but often stated in the negative: 'This plan is not exactly etched in stone.'"

It has an expression I've never heard. The opposite: "etched in sand. Flexible."