Suddenly, I heard a loud cracking sound that never stops. The stone grinder is normally noisy but I was talking about the unusually load weird sound. Some occurrences stopped after few second but others never, to the point that I need to take the burrs apart and inspect what was causing the anomaly. The culprit was nothing but hard stones, something like gravel and the likes. It was not only damaging the pair of stone burrs but adulterate the food as well.
If I have sorted the beans before roasting then there should have no stone passing through. This is one of the importance sorting and grading. There are more:
Large and small beans could be separated so that roasting could be optimize. Small roast faster and bigger roast slower. Letting them together in a batch results in uneven roast.
Empty beans could be removed. Roasting those is just a waste of time. They have no nibs and will got suck during winnowing.
Removal of broken bean. Those are likely to got burnt ruining the flavor. On the other hand those could still be of use for winnowing before roasting technique.
Weevil infested and moldy ruin flavor. They are really unwanted. The expected flavor change is far worst than burnt nibs.
Moist bean should be separated and dried further. They tend to roast slower. The same effect as roasting small and large beans together.
First benefit. I have never observed any stone producing loud annoying noise.