Igneous
Pumice
Felsic Volcanic Glass / Vesicular Rhyolite
Hardness: 5-6 Mohs (though brittle). Color: light gray, white, or cream. Luster: dull to pearly. Structure: Highly vesicular (honeycombed with gas bubbles), amorphous glass. Specific Gravity: < 1.0 (it floats).
- Hardness
- 5-6 Mohs (though brittle)
- Color
- light gray, white, or cream
- Luster
- dull to pearly
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Physical properties
Hardness: 5-6 Mohs (though brittle). Color: light gray, white, or cream. Luster: dull to pearly. Structure: Highly vesicular (honeycombed with gas bubbles), amorphous glass. Specific Gravity: < 1.0 (it floats).
Formation & geological history
Formed during explosive volcanic eruptions when magma with high gas and water content is ejected into the air. The rapid cooling and depressurization trap gas bubbles in the solidifying glass. Occurs across all geological ages, especially near subduction zones.
Uses & applications
Used as an abrasive in polishing, pencil erasers, and skincare (exfoliation). In construction, it is used for lightweight concrete and breeze blocks. Also used in horticulture to aerate soil and in denim 'stone-washing'.
Geological facts
Pumice is the only rock known to float on water until it eventually becomes waterlogged and sinks. Large eruptions can create 'pumice rafts' several kilometers long that can travel across oceans for years.
Field identification & locations
Identify in the field by its extreme lightness (low density) and porous, sponge-like surface. It is commonly found in volcanic regions like the American Northwest, Italy, Greece, and Iceland. Collectors look for pieces with unique inclusions or large pore structures.
