Diatoms are tiny eukaryotic algae. They are common phytoplankton. Most are unicellular|single cells]], but some form chains or simple colonies.
Diatom cells have a unique cell wall made of silica (SiO2). These walls (called "frustules") take many forms, some quite beautiful and ornate. They usually have two asymmetrical sides with a split (hence "diatom").
They are one of two great groups of single-celled organisms in the sea. The other is the coccoliths. Since diatoms are silica, and coccoliths are made of calcium carbonate, they do not grow in the same places in the sea. This was first discovered by T.H. Huxley in his voyage on HMS Rattlesnake, 1846–1850. Diatoms are also found in some soils and some waterways on land.