diatomaceous+earth

Diatoms are single-celled organisms. Their cell wall (called a "frustule") is made of silicon dioxide (SiO2). Most live in rivers, lakes, and oceans, but they can also live in wet soil. When the diatoms die, their skeletons slowly fall and collect at the bottom of the ocean.

There are over 100,000 species of diatoms (estimated).

Here are a few pictures of diatoms, lifted from off the web.