Each individual diatom possesses a cell wall known as a frustule that consists of overlapping halves, similar to a pillbox. Covered in delicate markings and intricate designs, diatom frustules are primarily composed of silica and are valuable for testing a microscope's resolving power. Reproduction of diatoms is usually asexual, the frustule halves separating and secreting new, smaller bottom halves. Since diatoms decrease in size each time they divide, periodic spore formations are needed in order for the diatoms to be restored to their initial dimensions.