Pumice

Mohs hardness: 6, Color: white to light gray or cream, Luster: dull/vitreous, Crystal structure: amorphous (volcanic glass), highly vesicular (porous) with low specific gravity.

Formed during explosive volcanic eruptions when highly pressurized, gas-rich silica magma is ejected into the air, cooling rapidly and trapping gas bubbles.

Used as an abrasive in polishing compounds, stone-washing denim, foot-care products (pumice stones), lightweight concrete aggregate, and horticulture.

Pumice is the only rock that can float on water because of its high porosity and buoyancy. Massive rafts of pumice have been seen floating in the ocean for years after eruptions.

Identify in the field by its extreme lightness (it will feel like Styrofoam compared to other rocks) and its sponge-like appearance with many tiny holes (vesicles). Found near active or extinct volcanic zones.