JohnD18 – It may be more beneficial for you to get a Green Bean Cleaner to remove your foreign materials. This will remove most rocks, twigs, string and other materials by feeding the coffee through a set of screens that separate “overs” and “unders” as well as an aspirator to remove dust. However, these are not very good at removing stones that are the same size as a green bean.
As for Destoners, these come in two basic types and both rely on the density differences between the stones and roasted whole bean coffee. The more expensive solution is the inclined vibrating table (similar to ensoluna’s photo). This works by blowing enough air through the table to lift the roasted whole beans, but not the heavier stones. The stones then move up the incline due to the vibrating table while the beans roll down the incline. These are more effective at removing stones and causes less bean breakage.
The less expensive Destoner and one that is more common with coffee roasters, is a vertical tube with air blown through to lift the coffee, but not the stones, into a receiving hopper. It is very common to see these mounted on the discharge of the Roaster’s Cooling Tray and most Roaster manufacturers sell them. It works by setting the airflow to be slightly more than the pickup velocity of the whole roasted beans, but less than the pickup velocity of the stones. However, it can increase bean breakage if the airflow is not adjusted properly since the beans accelerate as they enter the receiving hopper.