When I was a kid in France we would find stones in our lentils. Since I've been in the U.S. I've never found a single one, and I never bother rinsing legumes before using. Reading what you're saying, KYH, I realize I may want to start to!
The thing to understand with dried beans is that the majority of commercially grown ones are bush varieties which are mechanically harvested. So there is a greater chance of dirt, clods, and stones getting mixed in. Some packagers make a point of cleaning them, others don't.
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.
You should always do a cursory glance through any dried beans.
I don't think so, Pete. A cursory glance is how you wind up with stones on your plate.
If you're actually concerned, the best bet is to spread the beans on a flat surface, such as a cutting board or counter top, and physically move each bean into a bowl. Any rocks and most clods will show up that way. Then a quick rinse and you're good to go.
Rocks, twigs, and other debris are only a problem because most people won't take the time to actually sort through the beans.