Definition: Perlite is a form of volcanic glass that is mined all over the world. Horticultural perlite is made by exposing perlite to heat, which causes the trace water contained in the perlite to expand, popping the perlite like popcorn. The result is a very lightweight, white stone-like substance.erlite is considered a nuisance dust by regulatory agencies, which means that efforts should be made to control perlite dust. Eye and mouth protection are recommended to prevent the dust from being ingested. Perlite dust can aggravate existing respiratory conditions and cause eye irritation.