There is no stone known as a "cumber stone". It's possible you're thinking of a different word or have a misunderstanding.

Could you please provide more context or details about what you're looking for? For example:

* Did you hear this term somewhere? Where did you hear it?

* What is the stone supposed to be used for?

* Do you have any other clues about what it might be?

With more information, I can help you figure out what you're looking for.