It may seem tricky to use natural sharpening stones as you can only guess their grain size. Are they worth using anyway? I am sure - if compared to synthetic stones, natural stones perform better and give a more predictable result. There are many natural stones mined worldwide, most of them are ultra-fine used for finishing.
For a beginner I’d recommend Black Arkansas stone as it gives a very sharp and long-lasting cutting edge. The stone is universal and can be used with different steels.
Black Arkansas is a finishing stone, so it requires the blade to be accurately pre-sharpened. Please sharpen the knife on synthetic stone with grit J 4000 (F1200) first and then switch to the natural stone. In this case Arkansas will unlock its potential.