Diatom

Diatom, single-celled alga plant (class Bacillariophyceae) of fresh and salt water. Their delicately sculptured cell walls contain silica, and the two halves fit together like the halves of a pill box. Diatoms are important as food for many small animals, and when they die their siliceous skeletons sink to the bottom. In some parts of the world deposits many feet thick have formed over a period of millions of years. These deposits (there is one over 1,000 ft/305 m thick in California) are excavated as fuller's or diatomaceous earth. The fine silica grains are used in metal polishes and toothpaste, as they scour but do not scratch. Diatoms are also connected with the formation of petroleum deposits.