Pumice

Hardness: 5-6 (Mohs scale), Color: light grey to cream, Luster: dull to silky, Structure: highly vesicular/frothy (amorphous volcanic glass), Specific Gravity: 0.25-0.9 (floats in water)

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected rapidly. The sudden drop in pressure causes dissolved gases to expand into bubbles (vesicles) as the lava cools almost instantly. It is found in volcanic arcs and continental volcanic fields.

Used as an abrasive in soaps (pumice stone), cosmetics, and toothpastes. In construction, it is used as a lightweight aggregate for concrete and cinder blocks. It is also used in horticulture for soil aeration.

Pumice is the only rock that can float on water due to its extreme porosity. During massive volcanic eruptions, 'pumice rafts' can form on the ocean that are miles long and can float for years.

Identify by its extremely lightweight feel; it will feel much lighter than it looks. It is commonly found in areas of past or present explosive volcanic activity like the Cascade Range in the US, Italy, or Iceland.