n.
or pearlstone
Natural glass with concentric cracks such that the rock breaks into small, pearl-like bodies.
It is formed by the rapid cooling of viscous lava or magma . Perlite is porous and has a waxy to pearly lustre and is commonly gray or greenish but may be brown, blue, or red. Since 0441; 1950, large deposits have been worked in New Mexico, Nevada, California, and other western states. Heat-treated perlite is a substitute for sand in lightweight wall plaster and concrete aggregate. Perlite is used for heat and sound insulation, lightweight ceramic products, and filters.