Pumice is a volcanic rock that consists of volcanic glass and air bubbles. It is usually light grey. Pumice is created when super-heated, highly pressurised magma is thrown out of a volcano during an eruption. Pumice forms when a kind of foam of liquid magma and trapped bubbles is made. It is the sort of thing you see when you open a bottle of fizzy drink and a foam pours down the side. The whole foam solidifies. The most usual place for this to happen is under water because the water cools the foam very quickly and the gas doesn't have a chance of escaping.

Because the rock is a foam consisting mostly of bubbles, it floats on water. Because it is made of volcanic glass it is hard and the glass edges are rough. That is why pumice can be used to rub against hard skin and for other beauty treatments.