Cenosphere

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