The astonishing aspect of diatomite that can be observed by scanning electron microscope (SEM) is principally related to the highly regular arrangement of pores in the frustules. However, color is another consequence of this highly ordered porosity, but it can be revealed only by using transmitted-light optical microscopy (OM). Indeed, diatomite frustules are nature-made photonic crystals and for such a reason the diatomite frustules appear variously colored under transmitted-light microscopy observation. Diatomite frustules are a mixture of glass (hydrated silica, SiO₂.xH₂O) and air. Glass and air are two perfectly transparent optical phases characterized by very different refractive indices and in addition they are organized in the frustules in a very regular manner. Owing to destructive light interference phenomena among the light beams reflected and refracted at glass-air interface, the frustules appear slightly colored for the extinction of most light components contained in the incident white radiation. Since color strictly depends on the refractive index of the optical phase contained in the frustules holes, these natural materials can be used as optical sensors. Photonic crystals are quite common in nature indeed the colors of scarabs, butterflies, and other insect types are due to this optical phenomenon. This natural phenomenon has inspired the fabrication of many artificial devices, however to make very miniaturized photonic crystals is a quite difficult process. To copy this functional porosity of natural systems like diatomite frustules is a particular approach of materials science known as biomimetics.