The term "set stone" refers to something that is fixed, established, or unchangeable. Antonyms for this phrase include words like unstable, variable, flexible, open-ended, and uncertain. Rather than being set in stone, these things are subject to change, adaptation, and evolution. An idea or plan that is not set in stone is one that can be modified or revised according to new information or circumstances. Similarly, a person who is not set in their ways is open-minded and willing to consider new perspectives. In contrast, a person or idea that is set in stone is rigid, unyielding, and resistant to change.