pay rock

from The Collaborative International Dictionary of English v.0.48

Pay dirt \Pay dirt\, Pay rock \Pay rock\, etc.
   1. (Mining) Earth, rock, etc., which yields a profit to the
      miner. [Western U. S.]
      [Webster 1913 Suppl.]

   2. a discovery, especially after an exploratory process,
      which yields a profit or sought-after benefit; -- used
      especially in the phrase "to hit pay dirt".
      [PJC]