Sometimes you dig the rock, sometimes the rock digs you:

The sophisticated grinding tool deployed by NASA's rover Opportunity was apparently no match for a chunk of martian rock.

Even in failing you gain information. I'd imagine the RAT is pretty darned tough, and any rock tougher than it should be made of some interesting stuff. Of course, if they can't dig into it they may not be able to tell. Ah, the vagarities of robot exploration.