Calcareous phytoplankton plates. Coloured scanning electron micrograph (SEM) of parts of the skeleton (coccosphere) of the coccolithophore Emiliania huxleyi, a small marine algal organism. The coccosphere consists of plates (coccoliths) of calcium carbonate. These skeletons can make up the major component of limestone rocks, such as the chalk of southern England. E. huxleyi is found in marine environments worldwide.