Gold Label Hydrocorn Clay Pebbles are manufactured using a mix of the best qualities of pure clay. They are baked in an open furnace using high grade clean fuels. This guarantees absence of heavy metals or other contaminates. The 8-16mm uneven pebbles provide stability and the unique uneven shape is an ideal surface for roots and beneficial bacteria. The porous structure has a high water capacity and is suitable for both ebb n' flow and top irrigation systems.