The phrase "invaluable tool" means something that is extremely useful and important, to the point that it is impossible to value or measure its worth. Full definition
We have found that classroom contracts and student journals are invaluable tools for creating and maintaining a reflective classroom community. (facinghistory.org)
Good tests are invaluable tools in improving student learning outcomes. (eduflack.com)
It's an absolutely invaluable tool to learn about how expectant mothers view their open adoption relationship so you can know what to expect. (americaadopts.com)