Chameleons, copepods, fish, and many other organisms, use organic crystals for an astonishing variety of optical functions. These crystals are formed by specialized cells, in which remarkable control over crystal shape, size, and assembly is obtained using strategies beyond the state of the art in materials science and solid-state chemistry. While these cells were identified many years ago, almost nothing is known about their physiochemical and biological processes. We use biological tools together with physical and chemical methodologies to study the processes organisms use to produce either optically functional or pathological bio-organic crystals. Our main model organisms for these studies are zebrafish and medaka.