Pumice!!!
Pumice is a grayish white natural glass with many holes.
This rock is valuable in scoring, scrubbing, and and polishing material in both powered form and in Pumice stone.
     Deep within the Earth, molten rock, called magma contains water that is prevented from becoming steam by the great pressure that exists there. If magma reaches the earths surface where it is called lava, the sudden reduction  in pressure causes the water to turn into steam. When  lava solidifies and forms Pumice, countless tiny bubbles of steam are trapped before they can escape. Most of the volume of Pumice thus consists of air spaces. As a result, Pumice is so light that it will float on water!!. Powered Pumice is used as an abrasive by dentists and industry.

 
 

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