Perlite is an amorphous volcanic glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 that has a relatively high water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 content, typically formed by the hydration of obsidian

Obsidian

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...

. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial mineral

Industrial minerals

Industrial minerals are geological materials which are mined for their commercial value, which are not fuel and are not sources of metals ....

 and a commercial product useful for its light weight after processing.
When it reaches temperatures of 850–900 °C, perlite softens (because it is a glass).