Perlite is a type of very light, very porous pure white "stone" like soil additive used for soil aeration and drainage. Looks like small little white pebbles mixed in with the soil that is easily crumbled if crushed between your fingers. In most garden/potting soils it ranges from granular sized (like sand) up to about the size of a BB (maybe about 3/16" to 1/4").
