Diatoms are one-celled algae that make up a large part of the tiny, living matter in the oceans. They have external skeletons made of silica (like glass) in the form of two shells (frustules). Dead diatoms accumulate on the sea floor, usually leaving nothing behind in the fossil record but their frustules. Deposits of diatom remains can be hundreds of meters thick. The oldest fossil diatom remains are from the early Jurassic (about 150 million years ago). The first diatoms had radial (circular) symmetry and lived in saltwater. These centric diatoms diversified extensively during the next 100 million years (the Cretaceous). Pennate diatoms with bilateral (two-sided) symmetry emerged later, and colonized freshwater. As they continued to radiate into different habitats, diatoms reached their peak of diversity about 20 million years ago. Because they are sensitive to conditions such as water temperature, fossil diatoms provide clues to prehistoric climates on Earth.

Diatomaceous earth, or diatomite, is a flaky, whitish rock. It is actually a layers of the remains of algae called diatoms. Diatomite is highly porous, made of thousands of hollow, diatom shells, which may be ornamented with ridges or spines. Because it is lightweight, powdery, absorbent, and abrasive, it turns out to be useful in a variety of manmade things. Diatomite is an ingredient in toothpastes, insecticides, dynamite, polishes, and pet litter. Because it contains lots of air pockets, heat does not transmit well through diatomite. So humans also use it in fire resistant structures (such as the walls of safes). Its porousness also makes it a good filter, which is put to use in chemistry labs, the swimming pools, and drinking water treatment. Diatomaceous earth is even used as a bulk component in foods. If the label reads silica, you may be eating the remains of diatoms.