Highly reflective organic crystals are widely used in animal coloration and vision [1]. Guanine crystals, for example, have been studied for over 100 years and their role in structural coloration is well-established. We discuss the less-studied pteridine molecules which are used, in crystalline form, as colorants and as reflectors in vision. Isoxanthopterin crystals form reflectors in the eyes of decapod crustaceans which are used in image-formation, enhancing photon-capture and camouflage [2]. The isoxanthopterin crystals are arranged in nanoparticles, constructed from a shell of isoxanthopterin crystal plates arranged in concentric lamellae around an aqueous core. The reflectors are formed from dense assemblies of nanoparticles. Isoxanthopterin crystals are characterized by layers of planer H-bonded molecules, and the reflectivity of the material derives from the extreme refractive index (n=1.96) parallel to these layers. The reflectivity and scattering of the particles are enhanced by the alignment of the crystals and an optimized core-shell ratio [3]. We also present recent insights on the formation of biogenic crystals from studies of developing organisms. [1] B. A. Palmer, D. Gur, S. Weiner, L. Addadi, D. Oron, Adv. Mater., 30, 1800006 (2018). [2] B.A. Palmer, A. Hirsch, V. Brumfeld, E.D. Aflalo, I. Pinkas, A. Sagi, S. Rozenne, D. Oron, L. Leiserowitz, L. Kronik, S. Weiner and L. Addadi, PNAS, 115, 10, 2299–2304 (2018). [3] B.A. Palmer, V.-J. Yallapragada, N. Schiffmann, E. Merary Wormser, N. Elad, E.D. Aflalo, A. Sagi, S. Weiner, L. Addadi, D. Oron, Nature Nanotechnology 15, 138–144 (2020).