The metallic luster from the skin of fish is due to a photonic crystal system composed of multilayer stacks of cytoplasm and crystals. The crystals are described as thin (50-100 nm) plates of guanine, with no reference to their hydration state. We established through X-ray diffraction that their crystal structure is that of anhydrous guanine. We noted that their crystal structurefunction relationship is exceptional compared to other purines with similar molecular stacking of the crystal structure. These elongate in the direction of molecular stacking, in contrast to the biogenic anhydrous guanine crystals whose smallest dimension is in the stacking direction. On the basis of the known crystal structure of anhydrous guanine, theoretical growth morphology was calculated. These calculations predict crystals elongated in the direction of the molecular stacking. The exposed molecular plane of the biogenic crystals is the (102) plane, which is composed of densely packed H-bonded guanine molecules. It is known that the in-plane polarizability of guanine molecules is significantly higher than the direction perpendicular to the molecular plane, most likely causing anisotropy of the crystals refractive index. It is therefore conceivable that the unique morphology observed in crystals from the skin of fish is designed to enhance their light reflective properties.