Working in a jail setting, we go through lots of beans. We find (and are told about) plenty of rocks, though what I find more common are small clods of dirt that dissolve with a good rinse of the beans. Even at home, I will do a quick glance through the beans before cooking or soaking them. Lentils don't seem to have the problem as much, but I do find the occasional rock. You should always do a cursory glance through any dried beans. No company can guarentee 100% rock free dried beans. Even the USDA understands this and has a threshold for foreign objects (rocks, dirt, insect parts) in dried beans.