set in stone

Related to set in stone: smoother, touch base, revellers, stepping up to the plate

set in stone

Permanent. Typically refers to a plan or idea. We might get brunch next weekend, but nothing is set in stone yet. Keep in mind that these blueprints are not set in stone—they're just to give you a feel for the design.
See also: set, stone
Farlex Dictionary of Idioms.

set in stone

If an agreement, policy, or rule is set in stone, it is completely decided and cannot be changed. The exact terms of the scheme have yet to be set in stone. These are just preliminary ideas and nothing is set in stone. Note: Other verbs such as carved or cast can be used instead of set. These ideas are up for discussion — they are not cast in stone.
See also: set, stone
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • be carved in stone
  • be carved/set in stone
  • carve
  • carve (something) in stone
  • carve in stone
  • carved in stone
  • be set in concrete
  • up in the air
  • take the Browns to the Super Bowl
  • joke is on