Stones are eaten accidentally with steamed rice. A sudden loud and earth breaking explosion occurs in mouth while silently enjoying the dinner. Even soldiers or terrorist will experience shock when a piece of small stone collided between his upper and lower jaws.
I used to complain to mom every time a piece of crunchy hard stone cross my mouth. It always give me shock, headache and lost of appetite. Good if it’s broken, too bad if not. It needs a slow chewing to find and spit out a damn hard rock.
Mom’s reply. She can’t do anything about it. A whole day sorting out stones is not enough.
In restaurants, a stone in rice is a big shame. Just one incident is enough to drive the customer away. I have never experienced it in such places though. Maybe they have a strict policy on rice quality.
I am not familiar with rice postharvest operations and the following are just guesses.
Dust, hulls, leaves and straw pieces are removed by wind. Blowing air enough to drive away those contaminants but retain rice grains.
Large stones by means of wire mesh. Letting rice grains to pass through while retaining larger particles.
A computer eye. Passing the rice grains in small tube one by one in a fast phase. Any discolored object are treated as contaminant and tossed out. I am not sure if this kind of equipment exist.
During transport and retail, some stones are accidentally added to rice.
If the rice on table still have few small stones, blame it on inadequate cleaning operations and rough handling and not to soil where the rice grows.