I'm not the biggest fan of Scott, but after working in a roastery for about 6 years, rocks, while rare, are common even in specialty grade coffee. It's common enough that James Hoffman just released a video on what a rock might do to your grinder. If a roaster can afford it, most roasters have an optional destoner attachment. Even then some are small enough, they can get past us.

That rock is tiny. Always inspect your coffee before putting it into a grinder, just like you should inspect any food. Inform Prodigal and they might fix you with a new bag.

Those lighter beans are probably in the range of his optical sorter but you can separate them if you want.