igneous
Pumice
Pumice (extrusive volcanic rock)
Hardness: 5-6 (but brittle), Color: Light gray to cream, Luster: Dull to pearly, Structure: Highly vesicular/frothy (amorphous volcanic glass), Cleavage: None, Specific Gravity: 0.25-1.2 (floats on water)
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Physical properties
Hardness: 5-6 (but brittle), Color: Light gray to cream, Luster: Dull to pearly, Structure: Highly vesicular/frothy (amorphous volcanic glass), Cleavage: None, Specific Gravity: 0.25-1.2 (floats on water)
Formation & geological history
Formed when highly pressurized, gas-rich magma is explosively ejected from a volcano. The rapid cooling and depressurization trap gas bubbles inside, creating a sponge-like texture. Found in areas of explosive volcanic activity.
Uses & applications
Used in construction for lightweight concrete, in horticulture as a soil additive, in toiletries for exfoliating skin, and as an abrasive in polishing and stonewashing jeans.
Geological facts
Pumice is the only rock that can float on water due to its low density and high porosity. Large 'pumice rafts' can form after submarine volcanic eruptions, sometimes drifting across oceans for years.
Field identification & locations
Identified by its extremely light weight, abrasive texture, and hundreds of tiny visible cavities (vesicles). Look for it in volcanic fields or near active/dormant volcanoes like those in the Pacific Northwest or Mediterranean.
