Pumice
Pumice is a light-colored, extrusive volcanic rock so full of trapped gas bubbles, or vesicles, that it can float on water. It forms when gas-rich, silica-rich magma is violently depressurized during an explosive eruption, freezing the expanding bubbles in place as volcanic glass. Its extremely low density and rough, abrasive texture have made it useful to people for thousands of years.
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Pumice properties at a glance
| Type | Igneous Rocks |
|---|---|
| Mohs hardness | 5–6 |
| Crystal system | Amorphous |
| Luster | Dull to glassy |
| Fracture | Conchoidal, vesicular |
| Specific gravity | 0.25–0.9 |
| Transparency | Opaque, translucent in thin shards |
| Common colors | White, Light gray, Tan, Pale yellow |
| Rarity | common |
Geology & background
Pumice forms during explosive eruptions of viscous, silica-rich magma, typically rhyolitic or dacitic in composition, where dissolved volcanic gases expand suddenly as pressure drops, foaming the molten rock before it quenches into glass too quickly for the bubble-riddled texture to collapse. Because so much of its volume is empty pore space, a piece of pumice can be lighter than an equal volume of water and will float, sometimes drifting across oceans in large rafts after major eruptions and turning up on shorelines far from the source volcano. It is closely related to scoria, a similarly vesicular rock, but scoria comes from darker, less silica-rich, more fluid basaltic magma and is denser with larger, often collapsed bubble walls.
Major producing regions include the western United States, Italy, Greece, and Turkey, wherever silicic volcanism has been active in the geologically recent past.
How to identify Pumice
Pumice is unmistakable in hand specimen thanks to its extremely low density, pale gray, tan, or white color, and its sponge-like network of tiny, elongated vesicles, giving it a rough, abrasive feel and a dull, glassy luster where the bubble walls are thin. It streaks white and has a hardness around 5 to 6, though the whole rock crumbles easily because of its fragile, porous structure rather than any inherent softness in the glass itself. Its main look-alike, scoria, is denser, darker, and sinks in water, while pumice's ability to float is the simplest and most reliable field test available.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Pumice is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Pumice worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Field identification
- Construction or aggregate when suitable
- Landscape interpretation
- Education
Interesting facts
- Pumice is often light enough to float on water, occasionally drifting across entire ocean basins as rafts after major eruptions.
- The rock's countless tiny gas bubbles form when dissolved volcanic gas expands suddenly as silica-rich magma depressurizes during an eruption.
- Ground pumice has been used as an abrasive for skin care and polishing since at least ancient Roman times.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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