Start with a blank scene. Create a single rock. Move the rock vertically and watch it vanish polygon by polygon.
Also works with rock populations (not to be confused with fake stones shader). I think this problem occurs regardless of the direction the rock is moved.
(http://img.photobucket.com/albums/v78/jeff433/rockbomb.jpg)
The rock object is basically non-functional right now unfortunately. We're aware of the problem and will definitely address as soon as we can. In the meantime the Fake Stones shader works quite well for most needs and if you need single "hero" rocks you can always import one from another modeling program.
- Oshyan