Biogenic crystals are widely produced by animals for a range of functions such as forming iridescent structures e.g. scales, feathers, shells and reflective structures in eyes to improve vision in low-light environments by reflecting light back into the retina . The majority of biogenic crystals are made of guanine, a nucleotide in DNA (deoxyribonucleic acid). The origin of the optical properties of these biogenic guanine crystals can be explained by understanding their crystal structure, which consists of densely stacked hydrogen-bonded layers . The platelike morphology of guanine crystals formed in organisms leads to a high refractive index face with significant reflectivity for wavelengths in the range of visible light. Periodic variations in the refractive index (resulting from birefringence) leads to constructive interference, high reflectivity , and the ability to scatter light to generate structural colours .