Perlite is a volcanic rock which has been super heated into very lightweight, white, clean, expanded glass granules.
Perlite has similar properties and uses to vermiculite but, in general, holds more air and less water so needs to be irrigated more frequently. It is often mixed with vermiculite to improve its water holding capacity.
Expanded perlite is physically stable and chemically inert. The porous nature of the granules makes it lightweight to handle, holds good quantities of readily available moisture and has a strong capillary action. Since it is free draining, it is also well aerated.
Perlite is popular for use in propagating trays for cuttings or seedlings, usually when an overhead misting system is used. Because of its lightweight nature it should ideally be mixed with other medias when used in a flood and drain system to prevent it from floating.