Reticulite is an extreme form of pumice formed within high lava fountains; about 1,000 feet, (300 meters), high, or more. Dissolved gasses within the lava form bubbles which expand until they burst, leaving behind a fragile, honeycomb - like structure. In spite of its light weight, reticulite does not float. Drop a piece of reticulite in some water and it will sink much like chicken wire.
Photo: Warren Costa