The cell walls of diatoms are made of silica.
Their cell wall construction is known as frustule.
It consists of two thin overlapping shells that fit into each other such as a soap box.
When the diatoms die, the silica in their cell walls gets deposited in the form of diatomaceous earth.
This diatomaceous earth is very soft and quite inert. It is used in filtration of oils, sugars, and for other industrial purposes.