Perlite is not a diatomaceous earth but a light, electrically charged filter material that is manufactured by the expansion of vulcanic rocks (Rhyolite). It is composed primarily of aluminium silicate. Perlite has a fluffy structure which is due to the formation of tiny spherical beads from the original concentric shells.

It is found in Greece, Ireland, Sardinia and Corsika. During manufacture, it is ground and then heated to nearly melting point. As the enclosed water (between 2 and 4 percent) escapes, the Perlite expands by a factor of 20 and forms irregular flocks which are very light and porous.

The density of a loose heap is about 50 to 100 grams per liter and when it is concentrate by vibration the density is about 100 to 250 grams per liter. The size of the particles lies between 1 and 100 µm with the maximum lying between 10 and 20 µm. It holds together better than diatomaceous earth which is an advantage in vertical filters. This even allows the filter process to be interrupted.