Diatoms belong to class Bacillariophyceae of algae. These are called diatoms due to presence of an accessory brown pigment called diatomin. These are unicellular and non-motile and the motile stage is found in sperms. Their cell walls are silicified (called frustule), so after the death of the diatom cells the frustules accumulate at the bottom of the water body forming thick deposits known as diatomite or diatomaceous earth.