Koronis oval pumice stone:
The pumice stone is a stone of volcanic origin and of different shapes, almost always elongated or angular. Its components are joined together during the cooling of high viscosity magma.
It is usually white in colour and is resistant to cold, fire, weather and water-soluble salts. Although it is of medium hardness, due to its high friability, the abrasive power is very low, producing a very soft effect on the worked surface.