Perlite is some kind of volcanic rock, it's fairly solid ... vermiculite is also some sort of mineral derivative (I 'think' it derives from 'mica', but not sure) ... either way, perlite works by moisture adhering to it's surface, while the vermiculite actually absorbs the moisture, like a sponge ... I used vermiculite for years, but I like the perlite better ... :smokin: