A rock called diatomite is a sedimentary rock made up of tiny microfossils known as diatoms. These little guys have a shell that is made of silica. When diatomite is crushed into a powder it is known as diatomaceous earth. Diatomaceous earth is used as a filter brewing beer and in many other industrial processes. Just make sure you're using diatomaceous earth from a freshwater source. Diatomaceous earth from a saltwater source will ruin your beer!