Andreas mentions that some stones are so dense that apple juice won't soften them. He recommends Chanca Piedra. That is some powerful stuff. I have taken the tincture form of Chanca, and started sh*tting out some strange looking, greyish-white things in a few days. If they were not softened stones, I do not know what they were. It wasn't like I was eating something that would be grey & white. FWIW.