Coloured scanning electron micrograph (SEM) of the skeleton of a Campylodiscus neofastuosus diatom. Diatoms are small single-celled organisms found in every type of water source on the planet. The skeleton is made out of silica (silicon dioxide). Apart from being used as indicator of water quality, in their powdered form they are contained in tooth paste to increase abrasiveness or to increase water uptake in cat litter. They produce around 20-50% of all oxygen on the planet. They are usually between 0.01-0.2 millimetres in length.