< 150 µm, Volcanic rock, from USA.
Pumice is a very light colored, frothy volcanic rock. Pumice is formed from lava that is full of gas. The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes.
Pumice is so light that it actually floats on water. Huge pumice blocks have been seen floating on
the ocean after large eruptions. Some lava blocks are large enough to carry small animals.
The small cavities of the pumice stone have a very sharp shell which regenerates the sharpness of the pumice powder.
Pumice is ground and used in soaps, abrasive cleansers, pencil erasers and specially in polishes.
Finely ground pumice is added to some toothpastes and heavy-duty hand cleaners as an abrasive.
Pumice powder is ideal to polish surfaces like wood or metal. Wooden pores for example are filled with pumice during the polishing.