carve in stone
carve (something) in stone
To make something permanent and incapable of being changed, typically a plan or idea. We might get brunch next weekend, but we haven't carved anything in stone yet.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
carve something in stone
Fig. to fix some idea permanently. No one has carved this one approach in stone; we have several options.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.