Answer: Silica
Diatom is a group of minute algae. These organisms are unicellular and autotrophic. These are found in ocean, lakes, and soils. They can be observed in both marine and freshwater. The cell walls of these diatoms are made up of silica called as frustule. The silica wall contains of varieties of pores, minute spines, ridges and elevations which is specific for characterizing the diatom in specific genera.