Perlite is a volcanic glassy aluminosilicate rock whose root cause is acid lava, injected superficially or underwater during volcanic activities. The latter then cooled and solidified rapidly, under conditions that did not allow the atoms of the mineral to be placed in crystal lattice formations, which eventually gave perlite its vitreous form 

A very important factor that influenced the formation of perlite was the presence of water and various gases trapped in its mass at the time of cooling and solidification. The name comes from its texture characterized by the presence of grains, reminiscent of small pearls (pearls). Petrographically, perlite is a volcanic material. It has a characteristic perlite structure consisting of concentric breaks that look like pearls that are placed all over the glass, sometimes circular and sometimes in polygonal formations. 

This is also the pronounced characteristic of perlite. The aforementioned breaks are a phenomenon observed in acidic glasses.

More...