Nature is astonishing in the width and breadth of her supplied useful products. Man-made concoctions can be pretty neato, but even then, such stuff is concocted from nature’s ingredients. Other than dressing for market—crushing, grading, and packaging—the useful functionality of pumice needs no help from man.

Volcanic Origins

The genesis of pumice begins deep down underground in the fiery heart of an incipient volcano. In the crush of that incubator, a combination of relentless pressure and rock-melting heat infuses silica-rich rhyolitic magma with water. The swollen magma forces its way up through the confining stratum, breaking open old fissures or splitting open new ones. When that viscous, rock-shattering force meets the freedom of the earth’s surface, it manifests in an explosive, violent eruption, heaving countless cubic tons of water-infused magma high into the frigid atmosphere. Trapped water, freed from the caging forces of heat and pressure, flashes to steam, a frenzied unmaking⁰¹ that foams sticky molten rock into a glassy-stone froth that rapidly cools and falls to earth as pumice.

Pumice is Not Scoria

Scoria (from the Greek skoria, meaning rust) is a pumice-related type of porous volcanic rock, but differs from pumice in functionally important ways.

Scoria has a larger and thicker vesicular⁰² form factor: bigger bubbles, cavities, pits, and grooves, and thicker, heavier walls between the voids. That combo makes scoria a rougher, heavier, denser stone. The common red/brown/black lava rock sold for landscaping ground covers or used in decorative fire pits? Scoria.

Pumice has a much more finely structured form factor, one built on a microvesicular level. As such, pumice is more amenable to fine-tuned refinement. Getting even more into the weeds about it, pumice is a felsic volcanic glass, which is rich in lighter elements such as silicon, oxygen, aluminum, sodium, and potassium. Scoria is a mafic volcanic glass.

Volcanic Glass

Pumice is a volcanic glass, a “state of matter intermediate between the closely packed, highly ordered array of a crystal and the highly disordered array of liquid.”⁰³ Volcanic glasses are rich in silica, but being amorphous, or non-crystalline, they present no respirable hazard like crystalline silicas do, even when crushed to smoke-like dust. This glassy character-state also provides pumice with a useful hardness that, when combined with it foamed-stone form factor, creates a unique friability that finds wide usefulness, most famously as an abrasive grit.

Obsidian is a closely related volcanic glass, also silica-rich and amorphous, but without the vesicular form factor that makes pumice so versatile. Obsidian also forms from rapidly cooled felsic lava, but not explosively so, rather obsidian is formed (typically) within the cooling margins of rhyolitic lava flows.⁰⁴

Abundant and Sustainable

Pumice is found world-wide and abundantly so. It is simple to mine and widely amenable to fine-tuned processing, screening, and blending. A handful of commercial pumice deposits supply the bulk of the world’s modest pumice needs.

Etymology

The PUMICE name/identifier is derived from Latin, and is closely tied to its finely-foamed appearance. The Latin spuma means foam, and from that visual relationship came the Latin pumex, or pumice. Interestingly, pumice was once thought to be hardened sea foam, and so was called, for a time, Spuma Maris. There was also a point in history when pumice was referred to as lapis spongiae.⁰⁵ All serve as etymological evidence that pumice was known to and utilized by classical civilizations.