igneous
Pumice
Vesicular volcanic glass (primarily SiO2)
Hardness: 5-6 (Mohs), Color: Light gray to tan, Luster: Dull/Glassy (in thin shards), Structure: Vesicular/Amorphous, Cleavage: None, Specific Gravity: 0.25 - 0.7 (floats on water)
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Physical properties
Hardness: 5-6 (Mohs), Color: Light gray to tan, Luster: Dull/Glassy (in thin shards), Structure: Vesicular/Amorphous, Cleavage: None, Specific Gravity: 0.25 - 0.7 (floats on water)
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected into the air. The rapid cooling and depressurization trap gas bubbles, creating a frothy, glass-like texture. Typical of Rhyolitic or Andesitic eruptions.
Uses & applications
Used in personal care (exfoliating skin), industrial abrasives, lightweight concrete blocks, landscaping, and denim 'stone-washing'.
Geological facts
Pumice is the only rock that can float on water due to its incredible porosity. Immense 'pumice rafts' have been observed in the ocean following underwater volcanic eruptions, some spanning hundreds of miles.
Field identification & locations
Identify by its extremely lightweight feel and visible 'bubble' holes (vesicles). It is commonly found near volcanic activity sites like the Cascades (USA), Italy, and Iceland. Collectors should look for pieces with minimal staining and intact pore structures.
