I've not had any problem with a raytrace using stone. I changed a grass texture to one I had found (seamless) and that seems to be the beginning of my problem. I didn't put two and two together until I read a few other posts. I have had to go into the settings and turn off caustics under photon mapping because the raytraces started hanging up at 91%. Before I added that texture, I could run a raytrace with the above checked... I guess it was just to much to handle.
Debra