Rocks in Grain

After breaking a tooth on a rock in my groats, I switched suppliers and started going through the groats very carefully before cooking.  I’m finding about one rock per pound of groats.  The rocks range from one sixteenth to one eighth inch in diameter.

I sort my wheat less carefully before grinding than I do the groats and rarely experience any grit in the bread.  Either there are less rocks in wheat, or my mill is pulverizing the rocks.

With modern gravity tables or stoners being used in the mills, I had thought any rocks in grain should be quite rare.  What is acceptable?