Diatoms are unicellular algae. They live within glass shells which they build themselves from silica collected from their environment. Their shell has a special name, it's called a frustule. A solid glass shell can be a good thing if you're at the bottom of the food chain.

When a diatom dies it's frustule is left behind, like a skeleton. Some of the diatoms in these pictures are live, others (the colorless ones) are dead.