igneous
Pumice
Pumice (Vesicular Rhyolite/Andesite)
Hardness: 5-6 (Mohs scale), Color: light gray to cream/tan, Luster: dull to vitreous, Structure: highly vesicular (porous/bubbly), Specific Gravity: very low (less than 1.0, often floats in water).
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Physical properties
Hardness: 5-6 (Mohs scale), Color: light gray to cream/tan, Luster: dull to vitreous, Structure: highly vesicular (porous/bubbly), Specific Gravity: very low (less than 1.0, often floats in water).
Formation & geological history
Formed during explosive volcanic eruptions when pressurized, gas-rich molten rock (lava) is ejected into the air. The rapid cooling and depressurization lock gas bubbles in place as the rock solidifies into a volcanic glass.
Uses & applications
Used as an abrasive in soaps (lava soap), exfoliating stones, stone-washing denim, lightweight concrete production (pozzolan), and horticulture as a soil amendment for drainage.
Geological facts
Pumice is the only rock that can float on water due to its extreme porosity. Massive rafts of floating pumice have been known to drift across oceans for years after large volcanic eruptions.
Field identification & locations
Identify in the field by its extreme lightness (low density) and rough, sponge-like surface. It is commonly found in regions of recent or historic explosive volcanic activity, such as Italy, Greece, Iceland, and the Western United States.
