Fossil diatoms retain their intricately patterned glassy silica shell cases. Called frustules, these may be either rounded or elongated. Living diatoms are single-celled, algae-like protists. They make up much of the plankton in the marine and freshwater food chains. Their shells accumulate by the millions on the seafloor, eventually fossilizing to form a siliceous sedimentary rock called diatomite.















