Pumice

Hardness: 5-6 (Mohs scale), Color: light gray to cream/tan, Luster: dull to vitreous, Texture: vesicular (highly porous), Specific Gravity: < 1.0 (floats in water)

Extrusive igneous rock formed during explosive volcanic eruptions. It cools so rapidly that atoms are unable to arrange into crystals, trapping gas bubbles to create its characteristic foam-like structure.

Used as an abrasive in cleaning products and skin exfoliants, in lightweight concrete and cinder blocks, for stone-washing denim, and in horticulture as a soil amendment.

Pumice is the only rock that floats on water. During massive eruptions, huge rafts of pumice can float on the ocean for years, often serving as transport for small marine organisms.

Lightweight feel and visible holes (vesicles) are key indicators. Can be found in volcanic regions worldwide including the Aegean Sea, the Cascades, and Italy.