Pumice is extremely porous igneous rock and is light coloured which is formed during the explosive volcanic eruptions. This rock is porous enough to float on water surface for its low weight then eventually sink when water logged.
Pumice is often formed from the rhyolitic lava which is usually light coloured and are rarely formed from the erupted basaltic or andesitic composition.
Pumice has low specific gravity
The pumice is made from the gas escapes of the volcanic eruption. This makes abundant vesicles of the pumice to float on water surface. The pumice have thin walls which makes it light weight and the specific gravity of pumice is less than one that gives it the ability to float on water surface. When abundant pumice is deposited by the lava flow, sometimes large amount enough that makes an island which can float for a some years till the time when island is water saturated and it sinks to bottom.
Pumice have vesicles which is the indicator that it is made from the gas escaping process where gas trapped in the magma at solidification escapes which produces these vesicles. The cooling process is rapid because of the surface temperature so these does not have time to form crystalline structure.
Some magmas when are in the subsurface due to pressure of the overburden have dissolved gases in the magma. This is just like carbon dioxide in a sealed bottle of beverage. When this magma reaches the surface, the dissolved gases in the melt escapes as the pressure is released at the surface the temperature is low which rapidly solidifies the melt allowing them not to make any crystalline texture and the dissolved gases because of its low weight escapes the melt and forms vesicles.
Pumice is largely used in the blocks of light weight where for making blocks, concrete of pumice is made. The vesicles of the pumice allow them to trap air in between the pore spaces which makes them light weight.
The other use of pumice is as a decorative in landscaping and is used as drainage rock and soil in planting.