Lime plaster, coloured scanning electron micrograph (SEM). Lime plaster is a common building material made using mineral sands that are often composed of fossilised microorganisms. Here tiny fossilised diatom shells (small, round) can be seen amongst the sand and mineral fragments. Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Magnification: x330 when printed 10 centimetres wide.