Cenosphere
Cenospheres (Expanded Mineral Materials Containing Alumina and Silica) are a by-product of coal burning power plants and are lightweight, inert, hollow sphere filled with air or inert gas, The colour of Cenosphere varies from grey to almost white and their density is about 0.4 – 0.8 g/cm3 (0.014 – 0.029 lb./cu in), which provides them with buoyancy.