This does not sound like a school problem, nor mathematics. However, since it is an interesting question, I will provide an answer, at the risk of both answer and question being deleted.
Rock (stones, minerals) can be made into spheres in a very ingeneous way. We have a rotating drum very similar to a clothes dryer. Inside, we put the rock or rocks intended to be cut/made into spheres. We add coarse abrasives, like sand, crushed diamond... depending on the material the rock is made of. We then let the drum rotate for a long time until the rock is spherical. However the surface will still be rough. If we want that smooth, we have to replace the abrasive completely with a finer grade of abrasive, progressively, until the abrasive produces a smooth surface on the rock.
In fact, a machine based on the same principles is used to peel potatoes. Nowadays, most potato chips are done in factories and most of them not peeled.
