Vermiculite is expanded mica — a hydrated mineral heat-treated until it opens into soft, accordion-shaped pellets. It is the counterpart to perlite rather than a substitute for it: where perlite adds air and drainage, vermiculite holds water and nutrients and releases them back to the roots as the medium dries.
That makes it the medium growers reach for at the start of a plant's life — raising seed, striking cuttings, and keeping a potting mix from drying out between waterings. It is sterile, rot-resistant and roughly pH neutral.
If you want drainage and air instead, use perlite, or compare the full range of grow mediums.
