Type of rock

Pumice is pyroclastic igneous rock that was almost completely liquid at the moment of effusion (creation) and was so rapidly cooled that there was no time for it to crystallize. 

What does it look like?

Pumice is light in color and has a rough texture. It is made of volcanic glass that is very porous and frothy.

How/Where does it form?

Pumice is formed when lava with a very high content of water and gases is discharged from a volcano. As the gas bubbles escape, the lava becomes frothy (bubbly). When this lava cools and hardens, the result is a very light rock material filled with tiny bubbles of gas trapped inside.

Does it belong?

Pumice in this amount would normally NOT be found on an island like Thera (Santorini).