How to identify Pumice

To correctly identify Pumice (Pumice), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically white, cream, light gray, pale yellow, or occasionally light brown. Darker shades are rare but can occur.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous (glassy) on fresh surfaces, often appearing earthy due to porosity.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Highly vesicular (frothy, spongy), rough to the touch due to sharp edges of broken vesicles. Aphanitic (glassy) groundmass.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (glassy) with no distinct crystal form. May contain phenocrysts (larger crystals) of quartz, feldspar, or biotite, but these are typically minor.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is a volcanic glass.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in areas of explosive felsic or intermediate volcanism, typically associated with stratovolcanoes and caldera-forming eruptions. Occurs in pyroclastic fall deposits, ignimbrites (ash-flow tuffs), and as floating rafts on oceans after submarine or coastal eruptions.

Key Facts

Hardness

5.5-6 on Mohs scale (for the glass component, but crumbles easily)

Specific Gravity

0.5-0.9 (less than 1, hence it floats)

Crystal System

Amorphous (volcanic glass)

Color

White, cream, light gray, pale yellow

Luster

Dull to vitreous

Transparency

Opaque to translucent (thin edges)

Fracture

Conchoidal (on glassy parts), but typically irregular due to vesicles

Cleavage

None

Composition

Predominantly silicic (70-75% SiO2), similar to rhyolite or dacite, with minor Al2O3, K2O, Na2O, FeO, MgO, CaO.

Physical characteristics

• Crystal Habit: Amorphous, highly vesicular mass
• Cleavage Type: None
• Fracture Type: Irregular to conchoidal (on solid glass portions)
• Tenacity: Brittle, friable (easily crumbles)
• Luster Type: Dull to vitreous

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