Foamed rock aggregate

The chunkiness of this rock belies its actual lightness in the hand. What initially appears as gravel conglomerated, on closer inspection turns out to be regular, man-made ceramic spheres. Each one is a foam; as the clay is fired, the organic matter in the clay burns off forcing the granules to expand and form a honeycomb structure, while the outside surface fuses into a tough shell. They remind me of the egg cases of whelks that wash up on the beach.