Perlite is a volcanic product that is composed primarily of silica. It
comes in small beads 3-5mm in diameter (I've never seen it any other
size). Perlite is non-degradable, and non-compressable. It's primary
function is decreasing soil density and increasing aeration. It holds
little water and even less nutrients and is basically sterile and
pathogen free.
Most people use it to replace sand.
JMS