Permanently fixed or firmly established; not subject to any amendment or alteration. Often used in the negative. The deal isn't yet etched in stone, but we're confident it will go ahead as hoped.

What is written in stone?

Permanently fixed or firmly established; incapable of being changed. Often used in the negative. The deal isn't yet written in stone, but we're confident it will go ahead as hoped. See also: stone, written.

What does cast in stone mean?

Definite, fixed, as in We may choose to stay longer—our plans aren't cast in stone, or When Carl sets an agenda you can safely assume it's etched in stone.