iatomite: Diatom fossils
Sediments on the seafloor are the final repository for falling marine debris. Sediments that have accumulated a significant percentage of diatom shells are called siliceous oozes. Siliceous oozes tend to occur where diatoms are abundant in overlying surface waters. After ancient deposits of siliceous ooze have had time to cement and compact, the resulting diatom-rich rock is called diatomite. Occasionally, these rocks which originally formed on the seafloor become exposed on land. Diatomite, which is plentiful in the coastal hills of California, is mined to serve many useful purposes.

Since individual diatoms are very small, and the holes in parts of individual shells are even smaller, powdered diatomite is an excellent filter for liquids such as pool water. Diatomite is also used as an abrasive and a pesticide. The opaline silica that composes diatomaceous earth is jagged at close inspection. In fact, from the perspective of an insect, diatom shards are deadly pieces of glass which can pierce their tough outer skeletons. Just a sprinkling of dry diatomite makes it very difficult for an insect to pass over the ground. Ouch!