Diatoms are a group of algae (Chrysophytes) characterized by unique cell walls called frustules. Unlike most other algae, these walls are impregnated with silica (silicon dioxide), which makes them extremely hard, rigid, and resistant to decomposition. Because these siliceous walls are virtually indestructible, they do not decay after the organism dies; instead, they accumulate over millions of years to form thick sedimentary deposits known as diatomaceous earth.

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