This small wheel is a slurry stone with a diameter of 18mm (0.7''). It is particularly useful for bringing out the mud (slurry) on another larger CotPyr stone. You simply have to rub it circularly on the other stone and the mud will form in no time.
Finally, there is a special combined natural stone composed of two natural stones: A Pyrenees sand stone with a grain of 1200 is glued to a Belgian blue Coticule with a grain of 6000.
A great review of a CotPyr stone made by a customer :