Diatoms are interesting creatures.
They can be found most easily in home aquariums, especially in salt water tanks. This image came from a scraping of the glass inside my salt water aquarium. They are numerous in salt water tanks because of the tank water's high calcium and silica content, the hardness and the high alkalinity. This species usually looks like a carpet of rust-colored algae. Their exterior cases consist mostly of silica which they extract from the water in enormous quantities.