Perlite

Perlite is a hydrated volcanic glass, essentially obsidian that has absorbed water over geological time. It is recognized by its pearly luster and distinctive 'perlitic' texture: a pattern of curved, concentric, onion-skin cracks that give it a beaded appearance, which is the source of its name.

Unlike clear obsidian, perlite is usually grey, greenish, or brownish and somewhat dull. Its defining property is the 2-5% water locked inside the glass.

When heated to around 850-900 degrees C, that trapped water flashes to steam and the softened glass expands up to 20 times its volume into a frothy, lightweight white material, the form most people know commercially.

Perlite forms from silica-rich (rhyolitic) lava that cooled rapidly into volcanic glass like obsidian. Over long periods, groundwater slowly hydrates the glass, adding water to its structure and producing the characteristic concentric perlitic cracking as the glass swells and stresses.

The result is a metastable hydrated glass that still retains its volcanic origin but differs chemically from fresh obsidian by its water content.

Major deposits occur in regions of young silicic volcanism, including the western United States, Greece, Turkey, Italy, and Mexico, where it is mined in large open pits.

Look for a grey-to-pearly, somewhat glassy material showing curved, concentric onion-skin cracks (perlitic texture). It is less glossy than fresh obsidian and often lighter in color.

Hardness is roughly 5.5-7 and it has a vitreous-to-pearly luster with conchoidal fracture and white streak. The classic confirmation is behavior on heating: real perlite puffs up and turns chalky white because of its water content.

Distinguish it from pumice, which is naturally frothy and full of stretched gas vesicles (perlite is dense before expansion), and from plain obsidian, which is glossier, darker, and lacks the concentric cracking.

Expanded Perlite is a major industrial material. Because it is lightweight, porous, non-flammable, and inert, it is used in horticulture (the white specks in potting soil that improve aeration and drainage), as loose-fill insulation, in lightweight concrete and plaster, and as a filtration medium for beverages and water.

It is also used in foundry work, fireproofing, and as a filler. The combination of low density and thermal stability makes it valuable across construction and agriculture.

Perlite has little use in jewelry or metaphysics; its importance is almost entirely industrial and agricultural.