Pumice Identification Guide
Identify pumice, frothy volcanic glass, by its extreme lightness, abundant vesicles, pale color, and ability to float on water.
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What Pumice Looks Like
Pumice is a highly vesicular volcanic glass — a frothy, foam-like rock formed when gas-rich felsic lava is quenched so quickly that bubbles freeze in place. It is usually pale: white, cream, light gray, or tan, sometimes pinkish or pale brown. The luster is dull to silky, and the surface is packed with tiny vesicles (bubble holes) giving a spongy, abrasive feel.
It has no visible crystals — it is essentially glass — and is famously lightweight.
Step-by-Step Field ID Checklist
- Weight: Heft the piece — pumice is astonishingly light for its size.
- Float test: Place it in water; genuine pumice usually floats because of trapped air.
- Texture: Confirm abundant fine vesicles throughout, with a frothy/spongy look.
- Color: Note the pale, felsic coloration.
- Feel: It is abrasive — it scrapes skin (the basis for its use as a scrubbing stone).
Key Diagnostic Tests
- Density: very low; floats on water — the single most diagnostic field test.
- Hardness: the glass is ~5–6, but the rock is crumbly and friable.
- Luster: dull, silky, or glassy on bubble walls.
- No crystals: amorphous glass, no phenocryst-rich groundmass.
- Streak: white, with a powdery, abrasive feel.
Common Look-Alikes and How to Tell Them Apart
- Scoria: darker (red, brown, black), basaltic, denser with larger vesicles, and it sinks in water; pumice is pale, felsic, and floats.
- Vesicular basalt: dark and dense, sinks readily; pumice is light and floats.
- Perlite: gray, glassy, pearly volcanic glass that lacks the frothy floating texture.
- Consolidated volcanic ash/tuff: denser and gritty, without the continuous frothy bubble network.
The combination of pale color, frothy vesicular texture, and floating on water is essentially diagnostic for pumice.
Where It Is Found
Pumice forms in explosive eruptions of silica-rich (felsic) magma. Major sources include Lipari (Italy), Santorini (Greece), volcanic regions of the Pacific, and western US localities such as the Mono-Inyo craters (California) and the Cascades of Oregon. Floating pumice rafts can drift across oceans after eruptions.
Frequently asked questions
How can you tell if a rock is pumice?
Pumice is extremely lightweight, pale-colored, full of fine bubble holes, abrasive to the touch, and it usually floats on water — the floating test is the easiest confirmation.
Why does pumice float?
Pumice is a frothy volcanic glass riddled with sealed gas bubbles, giving it a very low overall density that lets it float on water until the pores eventually fill.
What is the difference between pumice and scoria?
Pumice is pale, felsic, very light, and floats, while scoria is darker, basaltic, denser, has larger vesicles, and sinks in water.
Is pumice a rock or a mineral?
Pumice is a rock — specifically a vesicular volcanic glass. Because it is glass it has no crystal structure and is not classified as a single mineral.
Pumice identified by the community
Recent Pumice specimens identified with Rock Identifier.
