Cenosphere

A cenosphere or kenosphere is a lightweight, inert, hollow sphere made largely of silica and alumina and filled with air or inert gas, typically produced as a coal combustion byproduct at thermal power plants. The color of cenospheres varies from gray to almost white and their density is about 0.4–0.8 g/cm3 (0.014–0.029 lb/cu in), which gives them a great buoyancy.