A coffee destoner is a machine that removes foreign matter such as gravel, metal debris, or even glass slag from roasted coffee beans so that beans is free of stones. This help commercial and industrial coffee roasting producer get more clean and safe products to end customers.
It uses the princeple of air gravity, the roasted coffee is lighter in density and is sucked into the coffee silo above the destoner, while the heavier density will remain. This equipment is a must for European coffee roasters. Its purpose is to consider the safety and hygiene of coffee users. It also protects the safety of the coffee user's grinder.
I don't think anyone would like to drink "mud coffee."
Green beans are agricultural products and it is inevitable mixed with gravel, metal and glass slag, because most of them are operated on the ground by farmers during the drying and collecting process, especially in Asian producing areas like Indonesia etc.