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. Perlite is prized for its moisture retention and aeration properties. It is naturally sterile and has a neutral pH. Horticultural perlite is used as a component in soilless growing mixes, a growing medium for rooting cuttings, or as a hyrdoponic growing media. Perlite 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. Perlite does not cause cancer, although some commercial perlite products may also contain a very small amount of quartz, which is listed as a carcinogenic substance.