Dropping stones onto your LEGO doesn't sound like a kind way to treat your expensive Technic parts; I imagine that small bits of stone and dirt will fall through a belt of Technic Chain Link or Technic Chain Tread , what with them being a chain rather than a belt. This will then lead to other bits of your contraption becoming polluted with abrasive gunk.
Akiyuki also built a brick sorter with a simultaneously tedious and exhilarating video of a machine sorting arbitrary LEGO parts; Photos and an extended discussion in Japanese at レゴのレゴ選別マシン その1). The conveyor belt perplexed me as I didn't know what it was made of. I thought it might be some kind of fabric or custom-manufactured belt.
Searching the interwebs pulled up this video of an effective conveyor belt built by a small child, comments to which make it clear the belt is paper. It's actively driven by rubber tyres at one end, and idles at the other (I'd suggest using 41531 Technic Cylinder 4 x 4 with Pin Holes and Center Bar as a crowned pulley rather than tyres).
This is the same material that Akiyuki uses as you can see in this low-angle shot up into the tensioning mechanism of the paper conveyor belt in Akiyuki's brick sorter.
Sticky-tape, paper and a smooth LEGO surface underneath the paper to ensure solid support (you don't need to use tiles; try a SNOT technique instead). Presumably you could up the paper to something more robust in the face of having stones dropped on it. You might want to also line the sides of your conveyor to prevent the stones scratching the sides.