Diatoms are a type of microscopic organism. They are a type of algae, specifically a common form of phytoplankton. They are autotrophs, meaning they produce their own food. As a result, phytoplankton are a common food source for small marine life. They utilize sunlight to make food via photosynthesis, similar to plant life. Due to their photosynthetic activity, phytoplankton produce the majority of the oxygen in the Earth's atmosphere.
What makes diatoms unique are their cell walls. While most cell walls are composed of various polymers of sugar and protein molecules, diatoms have a cell wall that is mostly composed of silica, or a hydrated form of silicon dioxide (the chemical comprising glass). This silica cell wall is called a frustule and is composed of two nearly symmetrical halves called bivalves that join together in the middle. This special type of cell wall is largely transparent. This is particularly useful for diatoms to allow the maximum amount of sunlight to enter into the cell to be used by the chlorophyll for photosynthesis. So the transparent cell wall helps diatoms to produce food more effectively.