Diatoms ( = through + "temnein" = to cut: "cut through") are a big group of eukaryotic algae. They are one of the most common types of phytoplankton. Most diatoms are unicellular, although some form chains or simple colonies. Diatom cells are encased within a unique cell wall made of silica (SiO). These walls, called frustules, take many forms, some quite beautiful and ornate. They usually consist of two asymmetrical sides with a split between them, which gives the group its name.