How to identify Pumice

To correctly identify Pumice (Volcanic glass (felsic to intermediate composition)), 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 light-colored: white, cream, light gray, pale yellow, or light brown. Darker shades are rare but can occur with more mafic compositions.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous (glassy) on fresh surfaces, but often appears dull due to its porous nature.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Highly vesicular (full of gas bubbles), frothy, spongy, and extremely lightweight. The vesicles are typically irregular and interconnected.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (non-crystalline) volcanic glass. May contain sparse phenocrysts (larger crystals) of quartz, feldspar, or biotite, but these are minor components.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is an amorphous glass. It exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Formed in explosive volcanic eruptions, typically associated with subduction zones or continental rift zones where felsic to intermediate magmas are generated. Found in pyroclastic fall deposits, pyroclastic flow deposits, and volcanic ash layers.

Key Facts

Hardness

5.5-6.0 (Mohs scale, for the glass itself, but due to porosity, it crumbles easily)

Specific Gravity

0.5-0.9 (can be less than 1.0, allowing it to float)

Crystal System

Amorphous (non-crystalline)

Color

White, cream, light gray, pale yellow, light brown

Luster

Dull to vitreous

Transparency

Opaque to translucent (thin edges)

Fracture

Conchoidal (on solid glass portions), but typically crumbles due to porosity

Cleavage

None

Composition

Silica (SiO2) content typically 65-75% (felsic to intermediate), with varying amounts of Al2O3, K2O, Na2O, FeO, MgO, CaO.

Physical characteristics

• Crystal Habit: Amorphous, highly vesicular mass
• Cleavage Type: None
• Fracture Type: Conchoidal (on solid glass), but generally irregular and friable due to porosity
• Tenacity: Brittle, friable
• Luster Type: Dull to vitreous

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