The perlite (from Latin «perle» meaning “a pearl”) is a rock of a volcanic origin.
A volcanic glass called obsidian is formed at the edge of the outpouring lava flow in the place of the primary contact between the magmatic melts and the surface of the Earth as the result of fast cooling (hardening) of the lava. Subsequently, the ground waters penetrate through obsidian providing for its hydration and forming of the perlite.
The perlite possesses a typical fine-dispersed concentric shell-like parting (the perlitic structure) due to which it is decomposed into round kernels (the pearls) resembling pearls with a characteristic gloss. The perlite is distinguished among other volcanic rocks by availability of the constitutional water (more than 1 %). The porosity value may attain 8-40 %.
The perlite may be colored in different shades of black, green, red and brown, brown and white. The perlite variations include an obsidian perlite (with obsidian impurities), a spherolytic perlite (with the impurities of feldspars), a tar stone perlite (homogeneous in its composition), glassy perlite etc.
In terms of the perlitic texture variations there are distinguished solid rock, striped, breccia and pumice perlites.