Any member of the algal class Bacillariophyceae (division Chromophyta), with about 16,000 species. They are tiny, planktonic (seeplankton), unicellular or colonial algae found floating in all the waters of the earth. The intricate and delicate markings of the silicified cell wall are useful in testing the resolving power of microscope lenses. The beautiful symmetry and design of diatoms justify their title jewels of the sea. Among the most important and prolific sea organisms, diatoms serve directly or indirectly as food for many animals. Diatomaceous earth, composed of fossil diatoms, is used in filters, insulation, abrasives, paints and varnishes, and as an insecticide.